i am wondering if the fast register save is affecting the re-enter_lp.bas include
Could be, but I don't have any PBP version even remotely up to date to test it with.

Could you post the .lst and .hex files that go along with that?

One thing I never see... the INTCON0.GIE bit needs to be set, and I only see that happening if there's a high-priority intr defined.
you can't have just a low-priority interrupt. If GIE = 0 then all interrupts are shut off.