Oh, just checked and I see that that the 1000 (or 1000A, but not 1000X) is a tubed design. It does have an AFC (automatic frequency control) circuit, and furthermore, it is user-switchable. You wouldn't happen to have the band selector set to "AFC OFF", I don't suppose? Anyway, this opens the possibility that there might just be something wrong with the AFC circuit, which might be easier to fix compared to tracking down a drifting component.