A common problem with Sony sets (and all others I suppose) is the CRT, they go low emission and run out of range.
Like most sets from that era and later, they have auto-greyscale - this means they continually adjust the greyscale to keep the picture as correct as possible. However, this adjustment only has a certain range, with the cathode voltage being reduced for any low emission guns - once it's been reduced as far as possible, and it's still not correct, then the sets shut down and display an error code.
You can often temporarily compensate for this by increasing the G2 (Screen) voltage a little, but it's only a short term solution.
It's easy to check - when it's actually working, monitor the three cathode voltages with a multimeter, they should be fairly similar, if one is much lower than the others, then it's likely that gun is low emission.