TV Black-out

The horizontal sweep circuits are at fault. There is an intermittent loose connection. It is likely a loose solder joint and it may be possible to spot if you have a magnifying glass and an eagle eye. The heavy lead that comes away from the side of the picture tube (CRT) will lead you to the Flyback Transformer area. Stay away from that wire - it'll knock the soup out of you if you don't know what it does - even with the power off. Don't try to re-solder everything in sight. You could do more harm than good. Examine the circuit board - you may be able to wiggle the cold solder joint and see it move where it shouldn't be moving. Good luck
TVGUY

a tap on the side tells me that it's a solder connection broken loose, most likely in the horizontal section. if you have experience with electronic soldering, you can fix it, otherwise take it to somebody that can fix it. thermal expansion and contraction(from turning the set on and off) eventually produces metal fatigue in solder connections, and the connection breaks loose, just enough that it sometimes works, and sometimes doesn't.

Unclejed is right. It is a dry solder. I suggest you unplug the TV set, wait for a few minutes to discharge all the existng High Voltage charged in the Electrolytic Capacitors to discharge, and then re-solder all the pins of the EHT / FBT (Fly Back Transformer) This is the area for most loose solder for your type of problem.