[John Bartley]

Some years ago I worked on a newer but somewhat similar Zenith AM/FM set which had similar symptoms in that it would start fine when cold and over a short time it would distort and stop playing.

Both the O.P.'s set and the one I worked on have slug-adjustable I.F. transformers and in mine I found an AM I.F. with silver migration issues. I fixed that I.F. transformer and it cured the problem.