On older machines like that, a CPU cooler isn't totally necessary when it's *idle*. But when you use an emulator on it, it uses 100% of the CPU the entire time, so you may want to try some cooling.
To see if it's really thermal related, find a fan. Could be a box fan for a window, fan for a power supply, whatever, as long as it moves air. Aim the airflow towards the CPU and start playing. If it does the same thing, likely heat is your culprit, and find yourself a CPU heatsink.