I'm not sure if it's heating problems but my PCs had the same fan setup from day 1 a couple years back... I even clean out the fans for dust regularly...
disable windows aero and see what happens
I get a problem, a black screen of lag when trying to open programs
This is exactly what happens when it's trying to change themes, so it has everything to do with your main problem
It takes two seconds to try out, unless you already have which hasn't been clearly stated yet.