I bought a 6870 about a year ago (I had a 8400gs at the time) and it worked epic. About 6 months later I used it in crossfire and it shuttered on BF3 ultra settings. I don't know why though, I figure it would shutter with 1 GPU cause I'm on 1080p but why did this happen? I sold the 2ng GPU because crossfire did not help me.
Basicly 1 GPU: high = 40-60 FPS (No AA) ultra 10-20 FPS
2 GPU: high = 60-80 FPS (No AA) ultra 10-20 FPS
I Don't Get This? :: The Cards Were 1GB Sapphire Radeon HD 6870's
More about6870 crossfire shuttering
Possibly memory issues with AA, catalyst profiles not up to date, driver issues, crossfire not ready for.BF3 at the time, CPU bottlenecks, micro stutter. We'll never know since you sold it before asking for help