Lock Data Rate 4 RC1

I have a 5 mile link with 2 29 db dual pol dishes. Signal Strength 65-70 dbm, 50 db SNR. How do I lock my data rate so it isnt jumping all over. I know you guys say if it isnt pushing that much data you dont need it. I am used to seeing 100/100 ccq. With this link I am seeing it flucuate from 97 to 60 and ping times used to be rock solid 1 ms across, I am now seeing sets of 20ms come through. Any help in my config with the new N cards would be appreciated. Thanks

Update

Antennas Pac Wireless 29 dual pol dishes, SR71-15 cards set N only WPA2 aes, Adaptive Imunity off, WDS off, Nstreme off, Both Chains, AMSDU Limit and Threshold 8129, HT guard ANY, Above Extension control, AMPDU Priorities NONE selected.