Belgand,
I have the same problem. To be more specific, my "white flashes" appear to be artifacts all over the TV screen. That is, not only in the viewing portion but in the black bars as well.

I basically tried the same troubleshooting as you that ended with the same result: wiring my DVD player and SAT receiver directly to my TV, which works fine. I also noticed that there was no rhyme or reason as to when it happened, i.e. they appeared on all media and channels. This is a hardware problem that needs to be addressed.