Ok.. we have a FIX
So what was the problem ? Actually it seemed that when going through the JAMMA Connectors +5V line it was not able to deliver enough current even though the voltage did not drop and smaller games would work because of less current draw.
A very odd thing that I couldn't figure out is I am measuring a short to ground.. like a 0 Ohm Short (never had that before) but the board works just fine - no hot parts and also the power supply stays cool.
My other MV-1C does not have that behavior and I think it also can't be some weird measurement because of some leftover charge of a cap.
But well if nothing get's hot and everything works how can there really be a 0 Ohm short to ground ?
So in one sentence - if you get a Pattern or white screen when trying to launch big games or multi carts you most likely do in fact have power issues (not enough current going through).
In my case the power supply was strong enough but not the path to the VCC lines.
So glad I could fix it even though the short to ground still leaves me clueless.....