A DIY Volvo owner MUST own a multimeter.
Your problem is intermittent, so I would suspect a bad connection. Mostly I would suspect the field wire to the alternator, then everything that connects the battery and charging system, including the battery-to-ground and alternator-to-starter connections. A bad connection can and WILL fry your alternator, so be very sure that the field wire is good, and that the alternator has good ground and good connection to the battery (via the starter, I think).
I replaced two alternators on a 1980 240, the third survived because I added extra ground and starter cables. These can go bad even though they look okay on the outside, don't really understand this but now I'm a believer.
Good luck.
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