It's very likely cold solder joints on the power supply board. Seek out the Pioneer 510 thread on here and search for Mr. Bob... he'll set you in the right direction.
I know what I am talking about...trust me. I've rebuilt a number of Pioneers using his advice. This was a very common problem among the Pioneers...once you fix it, it is likely to last for many, many more years. You do not need to pay a lot for the fix. The fix requires a resolder of all the points on the power supply board. After many years of thermal cycling, several high power contact points become erratic. It's a manufacturing problem that Pioneer had for a number of years with this particular board. If you catch it now, you'll be ok... if you wait and keep using the TV, you may damage the convergence or deflection board, which is a costly repair. Don't wait, and stop using the TV until you fix it.