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Shrinking picture is a voltage regulation problem in the switch mode power supply, Possibly a cap used in the feedback circuit. Or an over current condition on the B+. This is a very tech intense repair because there is no magic bullet. If it is shrinking on bright scenes then you can bet it is a regulation problem, bright scenes draw more current from the power supply causing this effect. I would not suspect the horiz output because the regulation for this circuit usually comes from the main switch mode power supply. One diagnostic is to monitor the B+ going to the horizontal output and see if it fluctuates. If so you can bet it is a cap or optical coupler in the switch mode circuit.
Sounds to me you have a cold solder joint ( loose solder connection ) on the printer circuit board around or on the vertical chip or on one of the vertical capacitors. You may also have a bad vertical chip or a bad vertical capacitor, but I doubt it since it did come and go. I'de get a soldering iron and some solder and touch up the connections. I will remind you that you need to always excersise saftey by making sure, before removing the back, unplug the TV from the wall, and use care not to come in contact with the large red wire going to the back of the picture tube as the picture tube can be turned off and still store lots of voltage ( up to 35,000 volts). Always stay safe and when soldering, use care not to run solder across places that will make a short.
Good Luck and I hope this helps
Ed Meshechek ( Radio Man )
It sounds like you have a problem in the 9 volt circuit which feeds the vertical output ciruit; CHECK C2P05 100 UFD 16 V OFF PIN 3 OF IC2P01 AND IC2P01 PART # 270P466020 (9-VOLT REGULATOR). BOTH ARE ON THE SIGNAL PCB.
This is not a repair that a novice should try, but if you call a factory authorized service center and have them come out and give you an estimate; AND tell them what the problem is, you will look know what you are talking about!
Most problems indicated by your description involved the Vertical IC. With your post "tap...pop back up" signifies a loose connection or to be more specific a cold solder. I suspect that no part(s) need top be replaced but would however require re-application of heat with the use of a soldering iron.
Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.