"1988 Miyata"

You didn't mention a notchy feeling. My first thought was an upside down bearing but I can't imagine that being unnoticed for 25 years. My second thought was the natural tendency of a headset would be to loosen itself which obviously isn't the case either.

I'm thinking some kind of foreign material may have gotten into the bearings. I'm going to agree with RoadGuy. Disassemble, clean, inspect and reassemble the headset. That should do it and it's a relatively simple task.

If your headset does feel notchy when you turn the handlebar from left to right, your headset is hosed and needs to be replaced.