Thanks for the detailed reply, and very helpful ideas.

The oil level was ok, and it has been chanbged since then.

When the smoke test was done, I sprayed some carb cleaner by the intake gasket, esp near the firewall and cyls 1 & 2. There were no revs or RMP jumps, so I think it is an ok gasket.

The idle when warm is about 1000 rpm if I remember correctly.

Performance is lousy. It stumbles wheh I give it gas. It also is getting much worse gas mileage because I have t floor it to go.

I had the middle muffler and pipe replaced, and I don't think it was a great job. It is a throaty sounding car now. Not sure how this could relate as there is only 1 O2, and it is before the pipe that was replaced.

I haven't checked the O2 with ohm metre.

I do not have a good Volvo map of the ground points. Is there one?

Which big plug and which wheel hump? Do you mean the one to the MAF?

I think with the help of the brickboard I am getting closer to the cause...