Make yourself feel good and learn a little bit about your car. You don't even need a manual. Many intermittent faults are loose grounds or grounds going loose. Get a couple of screwdrivers and some wrenches and sockets and start gently (!) tightening up any connection you can find where a wire goes to the body of the car or the engine block. Just snug them up. If you find a loose one that may be your problem, if not, you did a good thing anyway. Crawl under the car and look up, you'll be suprised at where volvo fastens ground wires. 70% of driveability problems are due to loose connectors or wiring faults. Anyone can trace a wire, if it looks frayed on the outside, suspect the inside. Good luck.
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Professionals have thumbs too.