I had the same thing on my 1995 850 GLT. Bought it new and in the first year went through 3 cam position sensors. A year later, when it happened again,it was recommended I use only synthetic oil and the problem was really caused by starting and moving the car a few feet (like out of the garage) and shutting it off. Doing this frequently can cause the valves to "stick" thereby causing it to run rough and throw the "check engine" light. As strange as it all sounds, I haven't had problems for about 2.5 years now since changing to synthetic oil. I can't explain it, but have heard this from more than a few Volvo gargages over the years. Just a thought, I'm not saying it is a miracle cure. Go figure?