Can you please specify the exact values (with units) for:
 Radl, dLradl, EcalERadius ?
Did you also verify that your final geometry setup does not contain overlapping
volumes (making the physical dimensions of a solid depend on the material
radiation-length is somehow... original and risky in this respect) ?

Yes, the same thing occurs when that slice is the only volume, besides Ecal (the
mother volume). I'm using only one material, so the radiation length is actually
a constant.

Radl               2.80839 cm
dRradl             1
dLradl             18
EcalERadius        4.58297 cm
EcalSRadius        2.80839 cm
EcalLength         50.551 cm

I still need some missing information in order to reproduce
your exact geometry and setup:
- Definition of the 'Plastic' material.
- EndRadius(2*Radl,EcalSRadius)
- EndRadius((2*Radl+Thickness),EcalSRadius)
- Possibly also the particle with position/direction you're shooting.
Is the rest (physics-list) the same as for TestEm2 ?

EndRadius(2*Radl,EcalSRadius)       3.00557 cm
EndRadius((2*Radl+Thickness),EcalSRadius)        3.00732 cm

  //Plastic
  density = 1.032*g/cm3;
  G4Material* PStyrene = new G4Material(name="PolyStyrene", density,
    					 ncomponents=2);
    PStyrene->AddElement(C, natoms=8);
    PStyrene->AddElement(H, natoms=8);


  density = 1.032*g/cm3;
  G4Material* Sci = new G4Material(name="Scintillator", density,
				     ncomponents=2);
    Sci->AddElement(C, natoms=9);
    Sci->AddElement(H, natoms=10);


    const G4int NENTRIES = 20;

    G4double TPB_PP[NENTRIES]    = { 2.0*eV, 2.1*eV, 2.2*eV, 2.3*eV, 2.4*eV,
    			           2.5*eV, 2.6*eV, 2.7*eV, 2.8*eV, 2.9*eV,
                                   7.0*eV, 7.03*eV, 7.05*eV, 7.07*eV,
    			           7.08*eV, 7.09*eV, 7.10*eV, 7.11*eV,
   			           7.12*eV, 7.14*eV };
    G4double TPB_RIND[NENTRIES]  = { 1.59, 1.59, 1.59, 1.59, 1.59, 1.59, 1.59,
   			     1.59, 1.59, 1.57, 1.57, 1.57, 1.57, 1.57,
    			     1.57, 1.57, 1.57, 1.57, 1.54, 1.54 };
    G4double TPB_ABSL[NENTRIES]  = { 35.*cm, 35.*cm, 35.*cm, 35*cm, 35.*cm,
    				     35.*cm, 35.*cm, 35.*cm, 35*cm, 35.*cm,
    				     35.*cm, 35.*cm, 35.*cm, 35*cm, 35.*cm,
    			             35.*cm, 35.*cm, 35.*cm, 35*cm, 35.*cm };
    G4double TPB_EMISSPEC[NENTRIES]  = { 1.0, 1.0, 1.0, 1.0, 1.0,
    				       1.0, 1.0, 1.0, 1.0, 1.0,
    				       0.3, 0.3, 0.3, 0.3, 0.3,
      	     		               0.3, 0.3, 0.3, 0.3, 0.3 };

    G4MaterialPropertiesTable *TPB_mt = new G4MaterialPropertiesTable();

    //TPB_mt->AddProperty("RINDEX",        TPB_PP, TPB_RIND,  NENTRIES);
    TPB_mt->AddProperty("WLSABSLENGTH",     TPB_PP, TPB_ABSL,  NENTRIES);
    TPB_mt->AddProperty("WLSCOMPONENT",     TPB_PP, TPB_EMISSPEC,  NENTRIES);

    Sci->SetMaterialPropertiesTable(TPB_mt);
    Plastic = Sci;

The physics list is the same except for the addition of optical photons and
their processes:

#include "G4Cerenkov.hh"
#include "G4OpAbsorption.hh"
#include "G4OpRayleigh.hh"
#include "G4OpBoundaryProcess.hh"
#include "G4Scintillation.hh"

void LXePhysListEmStandard::ConstructOp()
{
 // G4WLSProcess*  theWLSProcess = new G4OpWLS("OpWLS");
  G4Cerenkov*   theCerenkovProcess = new G4Cerenkov("Cerenkov");
  G4Scintillation* theScintillationProcess = new G4Scintillation("Scintillation");
  G4OpAbsorption* theAbsorptionProcess = new G4OpAbsorption();
  G4OpRayleigh*   theRayleighScatteringProcess = new G4OpRayleigh();
  G4OpBoundaryProcess* theBoundaryProcess = new G4OpBoundaryProcess();

//  theCerenkovProcess->DumpPhysicsTable();
//  theScintillationProcess->DumpPhysicsTable();
//  theAbsorptionProcess->DumpPhysicsTable();
//  theRayleighScatteringProcess->DumpPhysicsTable();

  theCerenkovProcess->SetVerboseLevel(1);
  theScintillationProcess->SetVerboseLevel(1);
  theAbsorptionProcess->SetVerboseLevel(1);
  theRayleighScatteringProcess->SetVerboseLevel(1);
  theBoundaryProcess->SetVerboseLevel(1);

  G4int MaxNumPhotons = 200;

  theCerenkovProcess->SetTrackSecondariesFirst(true);
  theCerenkovProcess->SetMaxNumPhotonsPerStep(MaxNumPhotons);

  theScintillationProcess->SetScintillationYieldFactor(1.);
  theScintillationProcess->SetTrackSecondariesFirst(true);

  G4OpticalSurfaceModel themodel = unified;
  theBoundaryProcess->SetModel(themodel);

  theParticleIterator->reset();
  while( (*theParticleIterator)() ){
    G4ParticleDefinition* particle = theParticleIterator->value();
    G4ProcessManager* pmanager = particle->GetProcessManager();
    G4String particleName = particle->GetParticleName();
    if (theCerenkovProcess->IsApplicable(*particle)) {
      pmanager->AddContinuousProcess(theCerenkovProcess);
    }
    if (theScintillationProcess->IsApplicable(*particle)) {
      pmanager->AddProcess(theScintillationProcess);
      pmanager->SetProcessOrderingToLast(theScintillationProcess, idxAtRest);
      pmanager->SetProcessOrderingToLast(theScintillationProcess, idxPostStep);
    }
    if (particleName == "opticalphoton") {
      G4cout << " AddDiscreteProcess to OpticalPhoton " << G4endl;
      pmanager->AddDiscreteProcess(theAbsorptionProcess);
      pmanager->AddDiscreteProcess(theRayleighScatteringProcess);
      pmanager->AddDiscreteProcess(theBoundaryProcess);
    }
  }
// set ordering for AtRestDoIt
}

the particle/direction is just default:

  G4ParticleDefinition* particle
                 = G4ParticleTable::GetParticleTable()->FindParticle("e-");
  particleGun->SetParticleDefinition(particle);
  particleGun->SetParticleMomentumDirection(G4ThreeVector(0.,0.,1.));
  particleGun->SetParticleEnergy(500.*MeV);

OK, the situation is more clear now.
The problem you're observing is eventually due only partially to the solid
you're using. The loop you get eventually happens in the optical processes
involved, and this problem has already been fixed in more recent releases.
I would suggest you to upgrade to release 6.2 where several important fixes
have been introduced in the optical processes (release 5.2 is rather outdated
in this respect...).
You can also find a patched version of G4BREPPolyhedra.cc here:
http://pcitapiww.cern.ch/geant4/source/source/geant4/source/geometry/solids/BREPS/src/G4BREPSolidPolyhedra.cc
which should appear soon in a patch to release 6.2.
You can also alternatively use the G4Polyhedra CSG-like version instead.
The new module for optical processes can also be found here:
http://pcitapiww.cern.ch/geant4/source/source/geant4/source/processes/optical .