[tom.seeger]

Actually. You should never change the mica or ceramic capacitors in the oscillator tuned circuit. These are almost never NP0 or C0G caps. They are almost always chosen by the original manufacturer to drift in exactly the right direction to counter any drift by the oscillators tuned inductor/transformer. So if anyone changed them for highly stable caps thinking they were doing a good thing, well they were not.
However I am not to saying this is the case in your radio. I have seen weak mixer/oscillator tubes do a bit of this, or quit altogether when they warm up.
See if you can determine if your oscillator caps have been replaced. Good luck.