Doom3 Stuttering

Ok, here's the problem: When I play Doom3 it stutters every 2-3 seconds or so. At first I thought this must have been something to do with poor performance, but after I enabled showfps, I am consistently getting between 35 and 60 fps at 1024x768 on High Detail setting. However, that every second and a half my fps drop down to about 25 for approximately a quarter of a second (just long enough to read the change in frames).

I did notice, however, that my HD LED blips at the same time the stuttering occurs. I have an Barton 2500+ oc'd to 2.1ghz, a 9800pro, 1 gig of PC2700, a soundblaster Audigy 2 Platinum, and a 120 gb 8meg cache Seagate Barricuda. My mobo is an MSi KT-600, and I've got a 431 watt enermax power supply.

I turned off all my applications other than Doom, defragged, and I really can't even begin to guess what might be happening. Does anybody have any other suggestions?

BTW, I have to mention that the game runs and looks great on my secondary rig with a Ti 4400 and a 1900+. Amazing what Carmack was able to do with this engine...

Try turning v-sync on and if that doesn't work, try using the new 4.9 beta drivers from ATI that is suppored to improve performance in opengl games by 3-6 extra fps.

Yup, I installed the 4.9's, but I haven't tried vsync on...

bump for a suggestion? It's not vsync. *Something* is causing the HD to be accessed every 2 seconds or so.

I fixed this on my rig by turning off ATItool. For some reason D3 did not play well when my 9800p was overclocked.
Try it.

By ATITool, do you mean that thingy that sits in the taskbar? B/C if so I already disabled that.

no, atitool is an overclocking utility.
If you've overclocked, try setting back to stock clocks.