MAME ROMs Freeze on Loading

Running RetroArch + EmulationStation on Windows 7 here. Everything in my setup works great but I can’t figure out my MAME issue - some ROMs start just fine and play great, but a good majority of them just freeze on loading at random percentages. Some make it to 70%, some 99%, some don’t even start counting up. I can quit RetroArch just fine and end up back in EmulationStation. I have all my BIOS files (though this happens with ROMs that don’t need any) and have tried each MAME core and ROMs from multiple version ROMsets. I’m stumped on this one - thought maybe I didn’t have enough RAM (2GB) but Dead or Alive++ starts just fine and then it freezes trying to load Turtles In Time. Help?