Magnavox TB100MW9 shuts down after a few minutes (or seconds)

I have three of these now with the same problem: They shut down after a few minutes of operation. Just curious if anyone has been able to fix one with that problem. I had one that started doing it a few months after I bought it. First it would run an hour or so, then just a few minutes, finally wouldn't stay on at all. I would appreciate any help (or a sevice manual copy if there is one). I did get into one and check all the caps with an ESR meter, but couldn't find any bad.