I have been chasing this problem for months as well as 3 Volvo shops and I left the hotshot dealer this morning when they were stumped as well. Here are the problems.
Car surges at idle and will stall. Shifting drops into default, High Gear, all the time then I have to stop turn off the key and restart. I do not have all the codes handy but will list them later. During diagnostics the car would kick the diagnostic computer out. I finally have a list, many pages. One thing is that there does not seem to be any communication between anything or the canbus. There is no 5 volt coming out of the CEM. No acceleration and sometimes when accelerating the car sounds very loud, throaty from the intake. No acceleration then at all. Sometimes I have some acceleration but when in full throttle it falls on its face. If this is wiring is it something I can see? or would it be covered and I would have to do continuity on everything? Since this is an 01 XC the cost through a dealer just to diagnose is far more than the value of the car. I thought of replacing the CEM to start (reprogram a used unit). See if that would cure the problem.