Sounds like an intermittent fault somewhere, a bad connection that came about during your recent PCV and engine work. I second Klaus's suggestion of checking over your electrical connections as there may be something right on the edge of connected/disconnected; that would explain why it was broken, worked for a bit, then broken again after moving when the mounts were replaced. Check everything over, electrical and vacuum connections, not just connections related to cyl. #3. You'll figure it out sooner or later.

I've been caretaker of a couple of cars that were finicky about OBD readers. The Mazda 6 would die as soon as the code reader was connected; the Honda will read codes at any time but won't clear codes unless the car is off and connected after being in key pos. II for a few seconds. So your Volvo may just be picky about being probed.