Greeting Volvonians
I have a manual 740gl which has done ~280k and still running very well - now at least after I solved a problem that had been affecting the performance for some years.
The car would run well for a week or two after service and then start to misfire so badly that it was somtimes unsafe to drive.
I had the mechanic/s look at nearly everything and they could find no fault so I gave it a try using basic reasoning. I started at the plugs and immediately had an insight. I noticed that when i changed the plugs that the top screw connection sometimes was loose. I tightened them, on one occasion and put them back in the engine . The car ran well for a few days and then started to misfire again.
I made sure that the neaxt set of plugs that went in had screw tops damn near welded on!
Problem gone. The old beast runs like a rocket, with 98 octane, of course.
Check the screw tops of your plugs as they may vibrate loose!
regards all

MurrayT