As soon as I read your subject line, I thought, Time to replace the set of brushes and voltage regulator. It's exactly what I experienced in both of my '93 240s.

It's easy to replace them yourself: you need a short Philips screwdriver, and a small mirror on a handle would be handy, too.

What makes it really easy is the removal of the sheet metal "stove" that surrounds the exhaust manifold*, so you can reach the back of the alternator more easily.
[ * I removed this permanently, along with the long foil tube that goes to the air filter housing, because I don't like the idea of "cooking" my AMM. But that's another story.]

My '93's original brushes from my cars were Bosch 1-197-311-022, EL 14V 4C

If you're inclined to stick with quality Volvo parts, you want Volvo p/n 3523710-6, $65.29, $58.76 with a VCOA discount.