Ok, let's see what we can do about this. First off, lets not pursue your overheating theory until we have some serious figures on the temperature instead of wild guesses. They lead to nowhere.
Your configuration isn't really up to the challenge to run Moh Airborne in the first place. Your 7400 is about the same or even weaker then the 6600 that's mentioned as minimal configuration for moh and your processor and lack off memory don't exactly make up for the slow videocard aswell. Most likely your software has been up in a jam and not your hardware. First try defragging your system and cleaning your registry and all with this 30 day free trial program
Linky
After you did that we will go on to step 2 and dig deeper into the issue of tweaking your system so it might just run moha at a reasonable fps.