[murf]

Hello fella's
Have an issue with a Zenith 10S464 that I just re-capped etc.
When powered up it plays excellent with sound quality and volume.
After about 15 minutes the volume will drop to about half of what it was when first powered up.
If I turn the volume up a little it reacts as it should.
Then after a while the volume will go up again for a while and then go lower again for a while.
Have checked the tubes and they are all in the acceptable range.
Seems to me like a tube issue.
Out of spec resistors were also replaced as well.
I did order a couple 6V6 tubes.
Could it possibly be an issue with the 6A8 or 6J5 tubes that test ok?
Any suggestions would be appreciated.
Thanks,
murf