Up until recently, my computer has been running perfectly normal when I played games, but now, whenever I try to play a game with decent graphics (eg, Oblivion, Neverwinter Nights 2, Left 4 Dead, etc), after about 5 minutes or less of playing, I begin to experience copious amounts of lag, about 1/10 of the frames are being shown it seems.

I don't have this problem with older games like Halo 1. But all of my better games are on Steam. It is Steam having problems, or is it a hardware issue?

I have a Windows Vista with a Nvidia GeForce 8400 GS video card.

Again, it ran fine until now. The last time I had problems like this, the problem was the video card melted where the fan-like thing was so it wouldn't turn, but I've since then moved the processor to a place where it would get better ventilation, so I doubt that's the problem.

Any help would be appreciated.

How hot is the GPU getting? Also you should post a dxdiag (preferably in spoiler tags)

When was the last time you updated your graphics drivers?

??? Sorry, but I'm a complete computer noob when it comes to actual hardware. Also, the back is only warm (about 70-75 F maybe)

I last updated my drivers about a month or two ago.

http://www.alcpu.com/CoreTemp/
http://www.techpowerup.com/realtemp/
http://www.techpowerup.com/gpuz/

Try updating your drivers.

Thank you, the second program reads
Temperature
46 C - 47 C
Distance to TJ Max
45 - 44

Also, I fail to see how updates my drivers would fix the problem since it was running perfectly fine for over a year, and the dxdiag says no problems found.

New info on my problem. The lag is periodic, it starts after a few minutes, then goes away after a few more minutes, then lags up again after a few more minutes.

Again, your help will be appreciated, especially when my problem is fixed.

Don't have any new processes running in the background do you, virus maybe?

How 'bout Xfire or some other chat program?

Nope, no processes or viruses, I scan for viruses daily and I don't use chat programs.

Simple question, when was the last time you used compressed air to clean out the dust build-up?

Could be pagefiling due to running out of memory maybe?

Yeah i would suggest checking memory

Personal opinion. /start
Its a really old video card, I had a 8800GTS (Alpha dog edition) and it svcked compared to a ATI 4870.
Its quite possible that the card is just in its death throws.
In fact if you wanted another one they are like $27-$40.

Maybe a bad connection? Maybe the hardware is going bad?
/end

Clean out the dust, check for new drivers.
Buy a new card? It seems from the specs (forgive me if I am wrong) of oblivion is pushing the limits for this card.

Updating drivers may cause the issue to go away, it is worth a try and very easy to do.

The curse is broken! I reinstalled the same driver I was using, and it seems the lag went away. Looks like you were right all along MorrowindFan, thanks.

I have a slight and funny lag problem on my laptop. I identified the cause but not the reason. Basically, the power plug is having a default (I guess due to overheat ?) and it allows the PC to get power but do not load the battery. Some small contact I guess is not triggered. If I move it and twist the cable and the plug, after a while, it will work.
Now, the funny part is that, when there is no charging of battery, the laptop is lagging, especially in graphic applications. When I use the battery alone, it works. When it is charging and properly plugged, it works.
Any idea how to solve that apart from getting a new power pack ?

If you have any electrical engineering skills and are not under warranty, you may be able to disassemble your power supply and reconnect/repair the fault.

If the above assumptions are not both true (you are not a skilled electrical engineer or you still have a warranty), you will need to either send it in for repairs or replace it. Laptops are not built to last (that's the tradeoff for the mobility).