It usually happens when something like A hero comes out of the altar, an overlord pops out for the first time and besides that it just happens radomly throughout the game. The games run well, but it's extremely annoying! this is my specs:

i7 920
Asrock x58 Extreme
OCZ3P1600LV6G
XFX Radeon 5870
750 W Corsair psu
Seagate 7200.12 500 GB
Windows 7