I had terrible heating problems when my mobo was mounted in the "front" 1/2 of the cabinet, right behind the coin door, random lockups, all that stuff. I added a few fans- a regualr 80mm case fan blowing IN a little below the board, and a 120mm exhausting out(it exited right below the CP) that helped a bit but I still got some lockups..
Since moving the motherboard to the "back" of the cabinet, remove the back panel, and there it is, below to monitor, I have had no problems with heat, and the only fan running is the one on the CPU. A much larger space made a ton of difference
Of course this is a P3/750mhz.. whenever I get around to upgrading I ma have problems again