Except for Art's mention of the knock sensor no one else has brought it up. It is possible it is defective and not compensating for the octane fuel you're using. Among other things you can do is try the highest octane fuel available. Art also mentioned carbon deposits - while I've almost never seen heavy deposits in a Volvo engine it can happen. When I've come across that situation I have successfully fed water (small amount slowly inducted through a vacuum connection at higher than idle speed). That process takes patience -- too much water at one time can cause hydro lock in the cylinders--water will not compress--and could lead to bent connecting rods.
I devised a water injection system when I built a very high compression B18 years ago--before 10 percent alcohol was common in gas. When I switched all my high performance parts to a B20 I found the pistons clean enough to eat off -- although I wasn't tempted to do that. -- Dave