... and then restarts.
Okay - so here's what's happening.
I start up my computer (Windows Vista 64-bit), and it will run fine for an indeterminate amount of time - sometimes 5 minutes, sometimes an hour or two. I have not noticed any correlation between this time period and what I am doing on it - sometimes it'll last for an hour while playing D&D Online, sometimes it'll start nigh-immediately. Anyway, after this initial period, it will begin to "hitch."
Everything will momentarily pause - the cursor, the game I'm playing, the web-browser, my Excel spreadsheets, whatever - for a second or two. If there is sound playing, it will replay the last second or so of sound in loop until the computer resumes. Sometimes, it will last longer, but the usual "hitch" is about a second. Then, my computer will resume working.
My computer will hitch repeatedly for an indeterminate amount of time - sometimes, five minutes, sometimes, an hour; again, no observed correlation yet between what I am doing and how long this will go on (e.g., surfing the PA forums the other day, it shut down almost immediately; playing D&D Online the other day, it let me complete a newbie quest [so about 30 minutes of playtime]). At some point, my computer resets as if I had pressed the reset button - back to black screen, power goes off, power comes back on, Windows reboots, etc.
When Windows comes back up, I will occasionally get a message stating that there was an error in the video card driver (recently installed a new Nvidia GTX 460, after my 8800 from several years ago started to go). I had recently updated to the August 9 release (280.26), and thought that maybe the problem was there. So, over a couple days (preparing for a new baby arriving in the middle of next month, so my troubleshooting has been slightly protracted), I variously rebooted into safe mode, uninstalled the current drivers, restarted in safe mode, installed the new drivers, ran a test (still resetting), and tried again.
Two days ago, I repeated the safe-mode process, except I reverted to the June release of the drivers (275.33). Last night, I logged into D&D Online as a test, and got about 10 minutes of standing around in a tavern before the hitching started, and about 30 minutes total before my computer reset. So, the previous drivers didn't solve the problem.
My case temps vary between the high-20s and the mid-30s, so I don't think temperature's a problem, but I am not ruling it out. The video card's got its own fan, obviously, which I have tried setting both to "auto" speed and "80% of Max" speed without measurable changes.
The hitching may have started before I installed my new video card; I can't actually recall when it started, but it has been getting worse; only recently has it started rebooting my computer.
So, PA Forums, what should my next troubleshooting step be?
For what it's worth, I've run full-scans from both Norton 360 (which I have up-and-running all the time) and Malwarebytes (which I only run as-needed), and both have come up negative. (Norton likes to find tracking cookiees.)