Where can I get new belts for AG-1980?
 

The tape stops moving by itself after a few minutes of playback, showing a still picture like a pause. I assume this is due to worn out belts. Where can I get new belts?

This is a K-mechanism deck, so the one belt it has is toothed and has an adjustable roller to adjust the belt tension. If the belt is loose maybe adjusting that is sufficient, it could be something completely different though. I think the mechanism underside where the belt is should be partially accessible from below, at least enough to check.

The Panasonic part number for the timing belt is VDV0235. You may find sources if you do an internet search.

However if it runs for a few minutes before it stops the problem could be something else. Does the display show an error code when it halts? Recommend you remove the cover and observe the mechanism to verify that it is a belt issue.