This is a physics problem:
The crank vent/fill pipe has positive pressure, the pcv line has negative pressure ... when one exceeds the other ... something's gotta blow.
Excess crank pressure from a "mature" engine is usually a ring/cylinder issue. Most likely the oil control ring(s).
The fix: hone / ring job
The cheap fix: Drain a quart, add a quart of Rislone get it hot and cross your fingers.
Otherwise: Hire it out for mosquito abatement.
-Hd
