I enjoy playing a few shumups on MAME (only if i can't afford the real pcb

) however, if theres a game that requires non stop button mashing, after a few minutes the buffer seems to overflow delaying the controls and ruining the game. If I quit MAME out to the front end, control is fine again. I am using an old keywiz PS/2 into my jamma harness. I don't think its the keywiz because it would still be loaded up after I quit MAME. I have searched around a bit on here but didn't see any relative posts. Any ideas?