mapping Genesis programs

I was wondering if anyone knows much about how the genesis programs are structured. When I load an image to my Gen from my MGH, there is a problem with the main (player controlled) sprites in most programs. That is, the player sprite in most (99%) games appears to be a random graphic garbage. The garbage is not always the same and not always to an extreme. This happens with images I have personally dumped or transferred from PC.

I am guessing it is a faulty part in the unit. so I opened up, naive as I am, and air dusted the MOFO. No improvement. But I noticed with shorter programs. (Berzerk and Double Clutch), no such errors occur. Strange. So now I suspect this only occurs with longer roms. Are the graphics for sprites stored in a particular area of a rom? more to the end; therefore, shorter roms are not affected? I don't know much about programming, but there must be some logic to this problem. or method of fixing it.
I have the same copier and I haven't been able to load and play any game, but I don't think the RAM is faulty because snes games did work (it's a snes and megadrive copier), so, I suppose it have something to do with converting roms or something.

I really haven't been able to play backups I made with the same unit, strange :\