Is this a recent devenopement or ongoing for some time?
This may sound odd, but you could check the output voltage at the plug. My dad was a t.v. repiarman, and he had a similar problem with an old set. Worked fine in the shop, wouldn't work right in the customers home. I was with him one day, and just happened to mention that. He checked the voltage, and was getting ~ 90VAC from the plug. Tracked it down to fauly wiring.
Couldn't hurt.
