Well here's the thing, what we would need to do on a DIY level, is use a meter and test ALL of the voltages coming from the supply, to ensure Vs is around 175 volts DC, and that Va is around 70. Also, check all power supply board capacitors, to make sure that none are bulged or leaking.
A shop would typically charge between $200-$250 for that type of repair.
If the supply caps and voltages look good, then we'd need to check the Sustain board heat sinks, to make sure that ALL are warm, and none are screaming hot or ice cold. In the even that they are, then we'd replace both Sustain boards (there is one on each side of the Plasma panel).
Average cost for this repair is verging on $450 parts and labor.
If the Sustain boards are working, and the supply voltages are good, then we'd move to replacing all of the Scan and driver boards- cost here is $250 parts and labor.
The last possible culprit is a failing Plasma panel, and this is the only part of the TV that is non-repairable as the panel costs more than the TV itself.
Unfortunately there is not a one shot diagnosis for this problem, and either of the culprits mentioned above can cause the problems you're having.
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