SNES screen flickers only when white

Hi!

I have a SNES with a weird problem. When ever the screen is all white the screen starts to flicker but as soon as something with another color appears the flickering stops. The problem appears on both my CRT and my plasma TV and with both RGB-scart and composite cables.

The system passes the SNES burnin testcart but flickers on the white parts there too.

Anyone knows what the problem might be?

 

Is the Composite Video unusually dark by chance? Or too bright?

 

It's a bit darker than my other SNES. But not something I have noticed before comparing them side by side. Here are some pictured:

http://imgur.com/a/VzWCu

Thanks!

 

That looks okay to me. So unless the TV is automatically increasing brightness levels, the composite signal should be fine.
The next thing you can do is measure the voltages across the board.

 

Are you using an official power supply?

Also, when you say it flickers, does it flicker on the whole screen or the edges of the image?