Breakthrough! I was trying all kinds of things and noticed, while jiggling the two ECC85 tubes directly behind the antenna connection, that the problem would come and go. I thought it was the tubes and found another pair, but as I was changing them out I noticed a pin on one of them was badly bent into its neighbor. So I straightened it out as best I could and replaced it in the socket, and that seems to have solved the problem. I've been listening several hours and it hasn't cut out!
Thanks to everybody, especially Gadget73 and Punker X, who had me focusing on the pins and the tubes on the front end (I had to look up front end). I always learn something on this site.
