need more information......as your GPU card is very light (6.4GB/s) bandwidth and only a 64 bit card, I think the problem might be that. However you must post all your specs so we can make an informed opinion. How much RAM? How big is the CPU? How many Watts is the Power Suppy? Motherboard full Name?
go here to check out your GPU against other GPU's
Win8.1/HAF X/CM 1000W/Z77X-UD5H/OC'ed 3770k @4.2GHz/32g G Skill/OC'ed GTX780 @1063MHz-6608MHz/Vertex3 120g/WD 500 Blk/WD 1T Blk
Win7/CM Centurion/650W/Z77X-UD3H/OC'ed 2500k @ 4.1GHz/16g Kingston/HD6870/Vertex2 90g/WD 500 Black
Win7/modded mid-case/725W/TA785G3+/Athone 2 x 4 640 OC'ed @3.442 GHz/8g Kingston/HD6870/WD 500 Blk/WD 1T Blk
doesn't keep me up nights, no siree