A systematic approach will find the problem. Besides something like the timing
belt jumping. When was the last time the fuel filter was changed? They will clog
up if ignored and cause running conditions like you describe. Including setting
codes in the motor cause it makes it run so bad. Whenever I get codes. I check
out the issues indicated. If I can't find anything wrong then I reset the computer. See if the codes come back. My mothers 90 245 has the POS ECU and it
always sets the check engine light. The computer runs the car fine it just doesn't think it is. So, we cover the check engine light with tape. Eventually,
I'll get a replacement ECU. In the meantime it runs great. Also the fuse in
the engine compartment caused running problems in this car till I redid that.

Another thing to check is the rpm sensor on for the ignition. It's on the
bell housing at the top and can cause weird problems. This is another
part that was intermittently causing problems then went bad in my mothers 90 245. I understand Volvo has gone through several revisions of this part.

I just replaced the head gasket on my turbo and could still see the factory
hone marks in the cylinders. At 277k miles! Unless you really abused this
motor there is no way that your rings are worn.

Dave 82 242ti 278k mi.