The most likely cause of this is a faulty shift micro switch. This is a common problem which is easy to fix and if I guy like me can do it and your mechanic doesn't know what the problem is or isn't willing to fix it, it might be time to find a new mechanic. Go to www.volvospeed.com and click on the maintenance link which will take you to an area called Bay 13. Under "Miscelaneous" there is a section (with pictures) on replacing this switch. Most likely, this will fix your problem. Also check out he other sections of Bay 13...you will be amazed at how much of this stuff you can do yourself. Good Luck.

Jim