I've had a MAF sensor go bad on me (not Volvo) and symptoms are similar to what you are describing. In my situation, the car would start fine, but after driving typically for more than an hour, the car would misfire, sometimes repeatedly and then go away. Typically when cruising on the freeway or accelerating up a hill. As usual the after the first time I took the car into the dealership they were clueless, but after 2nd time they figured it out and replaced the MAF.

My theory is that the MAF wouldn't allow enough air into the engine when warm so the A/F was too rich and would result in a misfire. When the engine is cold everything is fine because a little extra fuel is okay. I would check your exhaust tip and see if it has extra black soot, an indication that you're running rich. Furthermore, I think the problem is intermittent because the ECU will lean the A/F to compensate, thus the problem goes away temporarily. I have a friend of mine who is going through the same thing with his late model Kia Sportage. I suggested that he replace the MAF as well since the garage can't duplicate the problem.