I couldn't figure out what was causing this. I've just installed a whole bunch of new computer components, so naturally I suspect my video card, or RAM, or CPU, even though other graphically-intense games have been running smoothly. While running GPU-Z to monitor my video card, I waited for the stutter to happen and found that a load is actually taken off of my GPU - it drops from almost 100% (yet stable, mind you) to somewhere in the 70% range. It's like DIRT3 is losing priority for a few moments. It happens with shocking regularity: usually once every 1-3 minutes, which translates to at least once per race, generally.
With a little investigation, I'm positive I've found the culprit.
Games for Windows Live. You know, Microsoft's Steam wanna-be. You've seen it, that ugly program overlay with the slidy animations, that you're forced to log into, and paste your serial code, and probably don't have any friends on. Games for Windows Live, which seems entirely pointless since I downloaded the game through Steam in the first place.
Wanna know how I could tell Games for Windows Live (hereafter GFWL) is at fault? It's very simple. I've logged out of the GFWL service and the problem instantly vanished. Just like that -- gone.
I don't know who's more at fault, here: Microsoft for the programming behind their crappy GFWL "service," or CodeMasters for whoring out their game to MS for no apparent reason and requiring that you be logged in to save any progress in DIRT3, or Microsoft for coding such a crappy, intrusive game overlay.
Let me repeat that in case you missed it.
You can not save your game in DIRT3 without being logged into GFWL.
What the hell.
Why is Games For Windows Live even in this game? It's available on freaking Steam! In fact, the main reason I have this game is because I received a code for a free copy with my new AMD video card. The code was validated through Steam. What a surprise when I found I had to log into GFWL... even after Steam booted the game.
I've heard tell that reinstalling the GFWL program can fix this problem. I'll keep ya updated. Reinstalling the "Windows Live ID Sign-in Assistant" did not help.
DIRT3 is a great game, with really really fun tight controls, challenging AI, beautiful levels, gorgeous menus, and loads of diversity. It's actually one of the best racing games I've played in years, and I've been surprised at how good it is. But no game should suffer artificially because of this crap. I don't know the truth of why GFWL is in this game at all, but my gut tells me it all comes down to money.
CodeMasters, next time, pass on the DRM koolaid, eh? Your game's already on Steam. Leave it at that.