Sounds to me like alot of guessing/speculating by people inexperienced with YOUR Volvo. The fact they they first sold you a "tune-up" which obviously didn't fix the real problem tells me that they're guessing, not diagnosing the real problem.
Believe it or not, a good Volvo dealership master tech should be able to pinpoint the real cause in very little time in comparison as he'd have more real experience with your car. Most dealerships only charge 1 hour labor for proper diagnostics so diagnostic labor shouldn't be that expensive. My $.02