ProScan Picture Problem

I have a 27" ProScan model PS27113 dating from 1995 which has always been great. Suddenly, after about 30 minutes on, the picture slowly goes dark (takes about a minute). If I let it cool and turn on again, I can get another 30 minutes. The picture, when its on, is still as good as it ever was.

However, the sound continues unabated as long a I leave it on.

Is there a simple fix for this? Any ideas?

Thanks

Wes
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