you know what ROMs are, right? they're those chips on the boards in sockets. a very common problem I've seen with MK2 in particular is that if the roms ever come the slightest bit out of socket, the game does all kinds of BIZARRE shit.
it's a symptom called "chip creep", it's when the power on cycles of turning the game on and off gradually makes the chips move out of socket. they're not gonna entirely fall out or anything, but it'll make it where some of the chip legs don't make the best of contact and can produce any number of errors in the game, as the game's processor isn't able to read the software properly, does this make sense?
the solution in theory is easy, but for Mortal Kombat 2, the practice is not so much, because MK2 has a separate memory expansion board that plugs into it. basically what you want to do is press down on all these socketed ROM chips, use a thumb on each end and press down evenly. you'll hear them make a little creeeeeeeeek sound if they're slightly out of socket like I mentioned.

the hard part comes in removing the expansion board. not so much that it's hard to do, but that you want to exercise some caution when removing it, because it's connected with pins that can be EASILY bent if you remove it wrong.
there's some white plastic props that hold the expansion board in place. take a pair of needlenose pliers and squeeze the heads of these, making sure not to touch the PCB itself, and using your other hand, slightly pull up on the expansion PCB to pop it out of the props. once they're all free, assuming you have the board going long ways from left to right, and the expansion socket is at the bottom, gently (!!) rock the board up and down while trying to pull upward at the same time. doing this should remove the expansion board.
you should have 3 boards now: the main board, the memory expansion board, and the sound board. press down on all the socketed ROM chips one by one, and reattach the expansion board to the main board using my directions in reverse order. make sure all the connections are good and snug on these, cause it does make a difference.
ideally I place some of the smaller bubble wrap underneath the boards when doing this, makes it a little easier.
then hook it all back up and reinstall it inside your cab and see if the problem goes away! if not, then you have a board-level problem that you should probably not worry about fixing and just go out and get another working MK2 boardset. they're cheap on ebay. I overpaid for mine, like an obscene amount. they can usually be had for about $70-80.
let me know if you have any questions, or if any of my instructions are vague. I should probably put together my own guide for this with pictures, cause I've had to tell a couple people on here how to do this. :x