I remember your screen name so I'm going to assume that you have done all of the typical things. Bent derailleur hanger is a good suspect. In fact, on an old bike, they have very frequently gotten bent inward at soma time.

Shift into a gear combination that makes the hanger point downward. Then prop the bike up against something and look at the derailleur from the back. If the derailleur arm seems to point toward the rear wheel, that's it.

Park tool makes a device to measure and straighten it but, on a 7 or 8-speed, I have been successful just eyeballing the hanger into line. My technique is to leave the derailleur in place so the mounting bolt hole doesn't get ovalized and clamping a big Crescent wrench over the whole thing. Eyeball the derailleur arm until it is parallel with the cassette cogs and you should be good-to-go.