Sony KV-32XBR250 Trinitron TV

It sounds like two possible problems first power supply has over heated causing solder joint to come loose on printed curcuit board, or secondly,there is i black transformer at rear of tv,with wire that connects to the back of tube, this can sometimes fail ,you can try and reconect this but would advise not to do this yourself,as tvs hold high voltage charge for many years , and could result in death.you could take the tv to repair shop and tell them you know the problem,if they think you know what might be the problem, the may not rip you off hope this helps,,,


ps please be care full tvs can kill

Since I can't see your screen, I'll guess.

If the picture shrinks away in the vertical, fast or slow, it is in the vertical control circuit.

If the whole picture shrinks in both axes simultaneously, the high voltage circuit is the culprit.
This section is responsible for producing the accelerating voltage for the beam so if it is dropping out (quite common after years of use) there will be nothing to speed the beam to the phosphor to produce light.

Turn up the intensity when the picture drops out to see if you have any trace of light.
If not, it is the latter problem.

If you can still see a horizontal line that flickers a little (from video content) then it is the first problem described.

IF the flyback (HV) transformer is still available, it will cost somewhere between $25-$100 US (many lower cost sources have disappeared) plus labor.
Depending on whether the fault was the flyback or the high power transistor that drives it, there could be additional parts needed.

Since you indicated that it operates OK for a while, it could also be something as primitive as a failed solder joint and these can be tough to find (=$$ labor).