Whenever I play, the game will begin with my usual performance, but it rapidly degrades down to almost a "slideshow". Quitting the game and resuming also returns to a game with very low performance. These gradually-increasing spasms of low performance began as soon as I entered Goldenglow Estate during TG02, and affect even an old save from patch 1.3 (the character had also completed the quest).

Confounding factors might be last night's thunderstorm, but I am hooked up to a surge protector.

I've run antivirus checks, and defragmented the hard drive, and run a system memory check, with no problems. Does anyone have any idea what's going on?

You mention a thunderstorm. Have you tried other resource intensive programs to see if you experience issues with them?

It's hard to tell... maybe?

It's a laptop, BTW. It was only plugged into the outlet via the charge cable and an Ethernet cord.

You may want to do some stress testing, then. If it all worked before the storm, it sounds like something may have gotten a bit fried.

Well, what would I have to do to fix such an issue?

depends what component was fried...

usually replacing a component can help relieve the issue, however Laptops aren't designed for replacing parts (with the exception of RAM, HDDs, and the occasional Display).. this is why Laptops aren't meant for gaming (They are meant for "its gotten old, replace the whole thing", as opposed to "Its gotten old, Ill replace a few parts")...

take it to your local PC repair shop, tell them the problem you are having and ask them to preform a few diagnostic tests to see if any parts were damaged in a power surge..
If it is a part has been fried, and you are not "fortunate" enough for it to be one of the few replaceable parts on a laptop, you may have to buy a new machine...

Believe me from experience, when there is a severe thunder storm turn off all your PCs (or in laptops case, unplug them), Surge Protectors don't always serve the purpose they were designed for.

It depends on what's wrong, really.

For the CPU, you could run Prime95 for a while and see what crops up: http://www.mersenne.org/freesoft/
For the GPU: http://www.ozone3d.net/benchmarks/fur/ and/or http://unigine.com/products/heaven/ are good bets.

It sounds a bit weird but you can run 3dmark 2006 too, for an hour or so, if you don't get any freezes or ctd then you should actually try to look something else....

Give your laptop a full power cycle. The idea of a surge protector is to prevent damage for electronics but there may still be an excessive build-up of electricity in the laptop and I've found that this can cause various issues mostly related to booting up Windows. I've never seen it impact gaming especially during specific quests. You may want to provide more information regarding your mod list and if the issue is replicable on a fresh game save.

http://forums.toshiba.com/t5/Batteries-and-Power-Knowledge/How-To-Proper-Power-Cycle-Procedure-for-Laptops/ta-p/331424

Have you tried the Windows Check Disk, cause maybe something got corrupted.

If you dunno what Chkdsk is, here is a tutorial: http://www.w7forums.com/threads/how-to-use-chkdsk-check-disk.448/

Just make sure to tick "scan for and attempt recovery of bad sectors" too.