I'd take a look at http://www.techpowerup.com/downloads/2221/TechPowerUp_GPU-Z_v0.6.9.html and run GPU-Z to check whether your card is overheating and downclocking. If so, I would recommend pulling the clock speed down to a level where it doesn't do this. This constant downclocking is putting your GPU through massive amounts of thermal stress (orders of magnitude more than running at 99-100% constantly even if it is on the upper edges of the GPU specifications).

Also, clean out your GPU fan if you haven't. Dust might be causing overheating even at stock clock speeds.