Jamma Mega Drive - Picture is too dark

[GermanGameRoom]

Thanks to Porl I ordered the C1815 transistors and just installed them like in the tutorial

I was really pleased with the picture while playing Sonic.. after that I wanted to try Mega Turrican and after like 15 minutes of playing there is constantly happening something
like every 10-15 seconds the image gets almost totally white (you can still see everything) and is slowly fading back to normal (in like 5 seconds)

it's doing it with every game now.. does anybody know why this could happen?
It's like everything was fine until the system got a little work temperature or something..

I kept playing mega turrican until suddenly graphic glitches appeared as well.. the floor sprites where too high so I was standing in the floor.. and it was pixel junk all over the screen..

it's weird that it is just happening without touching the cartridge or anything..