I've had similar problems playing the SuperMod. I have no fix suggestions however, apart from upgrading your PC :-)

My guess is that it is caused by the fact that the SuperMod is also "super" in size. When playing it, the load times for both campaign and battles are much bigger than vanilla MTW, and I guess that the resource demand on the system is also higher. This can possibly expose programming weaknesses in the basic MTW engine, which does not surface under "normal" circumstances, especially on not-so-new PCs (my PC is a ADM Duron 1.2GHz, 256 MB RAM),

But as stated, it's just a guess.

I'm currently playing MTW XL2, which does appear a bit more stable for me, but it is also a smaller add-on.