I say clean the PCB's with water and dry with a hair dryer, it's the only way.
I'm not joking either, I had a KOF96 mvs cart with a background scrolling graphics issue, cleaned pcb with water and the graphics issue was no where to be found!
The 2-slot at the mall has this problem, though the whole screen will turn another color and start flashing and then goes back to normal after a bit. It probably just need cleaning (had a look once, the inside was filthy).