I did what my wife warned me not to do and purchased a Mortal Kombat 4 arcade machine off of a guy who seemed honest and had posted it on craigslist. Paid him extra to deliver it, but when he got it into my garage it ended up doing something weird with the keys. He fooled around with it for about a half hour, and finally got it working, claiming that there can be a host of issues that have to be adjusted after a machine is moved. I didn't want the poor guy to have come all the way out and then decide not to buy it, and it seemed to be working great when he left. I rebooted the machine, however, later that same day and the keys are now completely non-responsive in the game itself. If you go into diagnostic mode and do switch diagnostics all the keys respond and seem to be working, but when the game itself starts the keys do not respond at all.
I know this is a real long shot, but if anyone has any idea what could cause this I would greatly appreciate any advice! Trying to get this machine working for my kids (and so my wife doesn't kill me for getting ripped off on craigslist -- they guy who sold it to me told me tough luck and stopped responding to any communication).