Device seems to work, but resets after a few minutes of game play

Just received a new (to me) Game-gear and 12 games in the mail… The Game-gear looked to be in good working order and was obviously well cared for. I dropped in 6 fresh AA’s, and slapped in the Mortal Kombat cartridge… powered it up and played 4 rounds before it reset. I tried a different game, Batman Forever, and had the same result as soon as I reached the 3rd level. My Question is: Is this a Game-gear issue, cartridge issue, or just common bad luck with cartridge games?