Jerky/Choppy performance with 2x 4870x2

Here is my system:
Asus P5E3 Premium wifi,
E8600 4.4 Ghz,
4 gigs ram,
2x 4870x2 (quad crossfire),
Vista 64 bit.

I play at 2560x1600 resolution. Used both catalyst 8.9 and 8.10 beta.

Several games (Mass effect, Sacred 2 demo) give me the same problem. I get very high framerates, but every other second, I get jerky/choppy peformance. For example in both mass effect and sacred 2, whenever I move the character forward, I get smooth movement and next second I get a jerky/choppy effect. It is sort of like a hiccup. LIke for a second everything moves backwards and then forwards.

For both mass effect and sacred I am playing at max settings with 4axx. WHen I disable aa, the effect is ALOT less noticeable. The thing is I just read a performance analysis of sacred 2 done at legion hardware and they get about 120 fps with max settings AND 4xaa with a single 4870x2. I get choppy performance at SAME settings with 2x 4870x2! I asked them and they said they get perfectly smooth gameplay.

Other games like Grid, assassin's creed are perfect no problems perfectly smooth at highest settings and 8xaa.

Is this an issue with quad crossfire? Anyone know what is up? At first I thought this is microstuttering but that isn't a problem unless you have low framerates, which isn't the case for me.

Try and check your temps with riva tuner for each gpu and turn the fan speed up to full for both cards and then launch the game just to see if theres any difference, could be a temperature problem thats hindering the performance.

Hey thanks for the responses. The temps are fine on videocards, I am running fans at 75% for games and they don't get above 75 degrees. I am also using a 1200 watt power supply.

I am running two 300 gig velociraptors in raid 0, but don't know about the DMA settings. I will check in bios I can't find them in windows.

Here is the ram I have using:

OCZ, 4GB (2 x 2GB) Platinum EB Edition PC3-16000 DDR3 2000MHz CL (9-8-8-30) 1.8-1.85V SDRAM DIMM, Non-ECC

I am using 440 mhz for the FSB and dram frequency at 880. I think I changed some of the ram timings when I underclocked the ram. I will set them back to what they were.

Have you tried swapping the cards around?