Had a similar problem on 2000 V70. Intermittent misfires and CEL; always on cylinder #3 (P0303 code) even after swapping plugs and coils around. Had the car into two Indy shops who work on Volvos for a living; neither could diagnose this correctly and their only advice was to "monitor," which I did until the #3 misfire occurred again, and again, and again.

Turns out through my own troubleshooting that it was the #3 coil receptacle housing that needed to be replaced and repinned. After so many times unplugging and plugging the wires into a coil, the receptacle housing clamp weakens or breaks and the pins no longer make good contract with the coil terminal blades. From what I've observed, most troubleshooting about this problem is limited to swapping coils and plugs around, and not examining this connector. (Same as the Indy shops, apparently).

Here's a writeup about replacing and repinning that coil receptacle housing, which is a part available from Volvo. The wires must be removed from the old housing and repinned into the new one:

https://www.brickboard.com/AWD/volvo/1663046/V70-XC70/coil_receptacle_extraction_tool.html