How did the spark plugs look when you removed them? I just pulled the plugs from a '92 than were badly worn - but clean.
How did the oil look?
We had a '90 744 TI(150K) that had three cylinders with poor compression - after 30K with 3333K oil changes (3333K so that 10K services lined up) the compression came up in those cylinders.
Your car's computer may have accommodated older components and may need time to adapt to the new parts.
I have also had a tune-up sharpen up a bit after a thousand miles or so in older cars.
Drive the car for a couple of thousand miles or so and check it again before you throw more parts at it.
If the results are the same then, have an experienced Volvo tech look at it.
One thing that you might do, is to unplug any connectors that you can, clean them, add dielectric grease, and zip tie any that you can. I did one per week so that I know where to look if there was a problem.
Cleaning grounds is a good idea also.
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