Curiously, I just had a similar experience with my 1980 242DL and B21F. A few months ago I had installed a new distributor, distributor cap, rotor, plugs, plug wires. So, I checked all electrical connections, and all seemed OK. Had a friend, retired Volvo mechanic with 45 years experience, come over and double check what I saw. He thought the main fuel pump fuse was a bit loose in its holder, so we cleaned connection and tightened that up. While our two sets of eyes and hands were checking wiring under the hood, we thought that the coil wire (which had NOT been replaced along with the plug wires) seemed unacceptably loose at the distributor. Put in a new coil wire, and all seems fine now for a week or more. New coil wire was soooo much tighter than the old which basically slid right out of the distributor with very little resistance.

All of this makes good sense, but always a question with very intermittent electrical glitches.