Sync symptoms

[W9RAN]

Doing my first TV restoration - a Philco Predicta Princess. The good news is, after replacing some electrolytics and a few other caps, I was rewarded with a good picture and sound. Problem is, it doesn't stay that way for long - after about 15-20 minutes of operation the sync starts to go. The problem seems to affect both horizontal and vertical sync, It starts with tearing along the side of the screen but soon just becomes unwatchable and neither horiz or vert controls help. However the video brightness remains unchanged and the sound is still fine, which reinforces my theory that the problem is in the sync circuit.

If I turn the set off and back on again after a short time, the problem remains just as bad as it was. But if I let everything cool off for 30 minutes or so, it'll work fine again, until the problem recurs after 15-20 minutes.

Tubes all checked good but that doesn't mean anything. I haven't found any overheated resistors and don't see any likely suspect caps in this part of the circuit. I thought perhaps the symptoms would sound familiar to someone with more experience - thanks in advance.

Bob