Sounds like the PVC material exiting the die is too cold?? perhaps not fully fused??-----what is the exit melt temperature of PVC?? Should be in the 185-190C range. Is the material "cheesy" (partially fused) at the vent?
If you can disclose your formulation, along with extrusion conditions of heats, RPMs, flood or starve feeding, amps, etc. as well as the type of pipe (thin wall, heavy wall)----I'll try to offer some help.
(What do you mean by "reversion" test----I've not heard that term before)