Offline gaming lag -- Need some help

I'm starting to get back into gaming but unfortunately I'm having some issues with playing UT2004. Sometimes the game runs just fine and dandy and I don't notice any lag at all. However, at other times I get lag all of a sudden. It's the kind of lag where where doesn't seem to be any FPS drop at all; instead, there's just a ~2 delay every now and then between when you trigger a command and when that command is dislpayed. For example, I would be running forward and then turn to the right. When the lag is there, what happens is that it'd show me moving forward the whole time and then the game would stop for a second, then all of a sudden it'd show looking to the right (Roughly 2-3 seconds after I moved my mouse to look to the right). This issue seems to happen sporadically and seems to be independent on what's on screen (When the problem is there, it seems to happen periodically regardless of whether I'm in the outskirts of the map or if I'm in the middle of all the action). It also seems to be independent of the map/bot number as well. I've had the lag start of all a sudden with a certain map with a certain amount of bots. Then I'd stop and play again sometime later in the day (exact same map/bot number), and then the problem would be gone all of a sudden and the game would play perfectly. But simply rebooting or turning the computer on/off doesn't always seem to fix the problem.

Any idea on what might be up? I first thought it might just be my computer specs (Which are admittedly crappy for gaming), but the problem occurs sporadically (When it doesn't happen, UT2004 plays perfectly for me). There doesn't seem to be any difference in resources/system usage between the times when the problem is there and times when it isn't there (I play the game right when the computer boots up in both cases). I completely formatted my harddrive and re-installed XP/UT2004, but that didn't help. Any ideas?

I haven't tried many other games yet, but out of the handful I've tried it's only happening with UT2004 so far. I have the latest path for the game (3369). I should have the latest motherboard/video drivers from when I formatted my computer a couple days ago. AA/AF are on the "application" setting for my graphics card (and I didn't see any option for them in UT2004, so I'd say they're both off). I'm not sure how to set the vertical sync.

I'm thinking it's a software issue as well since I've had the problem happen to me when playing at 320 x 240, but have had it not happen to me when playing at 1024 x 768 (Same map, same number of bots).