It's certainly worth checking the PCV valve as an easy fix, but there is a well-documented head gasket issue with the K-Series engine, and back pressure through the air intake system could unfortunately also be a symptom of this, especially if the car has previously had a slight misfire under load. It's worth getting the system pressure tested by a mechanic before exploring further. The good news is that if you want to keep the car going, it's not a massively expensive fix and parts are freely available.