That's a bit strange.

I run Inventor on my Dell Dimension E520 (Intel Core 2 Duo CPU, 3GB DDR2 RAM, running at 2.13GHz on Windows XP Media Center) and I can have several parts and assemblies running at once as well as a few tabs open in Firefox at the same time with no lag, but when I play Garry's Mod, if I spawn too many props (>25) it lags like crazy.

I think it's probably the computer's reaction to continued use, possibly heat?