Hey folks,
 
About 6 months ago I noticed a drastic loss in performance from my PC. Most notably while gaming but also while browsing the web etc. Performed tests on CPU w/ Prime 95, performed tests on GPU with various utilities, as well as on memory and hard drives.
 
Did clean installs of both Windows 7 x64 and Windows 8 x64 (and I mean clean, no data or even backups carried over), I was still unable to identify the bottleneck/defective hardware that was causing the performance loss. (As side note, the performance loss was huge, for example most 2010 PC games became unplayable due to low framerate just to give you an idea).
 
PC Specs
 
Intel Core i7 950 @ 3.06 (Torture Tested)
eVGA Geforce GTX 680 (Standard Clocks) (Tested with various utilities such as Furmark, also benchmarked with Futuremark and scored similar to other pc's w/ same spec)
eVGA x58 Classified MB
6GB Mushkin PC3-12800 (3x 2GB Triple Channel) (Memtested)
320 GB WD 10K RPM SATA (Tested with various utilities, verified no errors from SMART)
Corsair 850w Modular PSU
 
Now here is where things get interesting. Earlier today I decided to disassemble the entire PC and clean it top to bottom from all dust etc, and Walla! It worked. Performance back to normal yay... unfortunately 8 hours later it regressed back to the aforementioned problem. Have monitored temps on all pertinent devices and hardware pieces and nothing is spiking or performing outside of comfortable let alone maximum acceptable values.
 
I am truly at a loss. I do not know how to identify what needs replacing (if anything) and my 3 year RMA for my GPU runs out on April 5th so I don't have much time if it ends up being the Video card.
 
Thanks in advance for any advice.

Does reseating the GPU bring the performance back? How about unplugging the PSU from the wall then pressing the power button for 30 seconds then replugging and powering up?

How filled is the HDD? How is your paging file setup? What are you using for firewall/antivirus/antimalware programs?

I have discovered something and have reproduced it 3 times now under identical circumstances.
 
In the aforementioned post, I said that the performance came back after I disassembled/reassembled my pc, and then after about 6 hours it reverted to being problematic. I was assuming that something I did in regards to the reassembly caused things to work properly for that 6 hours, but it has nothing to do with that. That disassembly was the only time my PC has been off for more than 30 seconds in over a year.  
 
Last night I left the PC off just to test, and upon waking in the morning and booting it up. Tada, running amazing, played Elder Scrolls Online all day at a nice 65 fps average (Maxed settings at 1920x1080). Unfortunately 7 hours after I started. While in game actually, my FPS dropped immediately from 60 ish to 20ish with absolutely nothing occuring. I was actually eating something so when the drop happened I was not actually moving my character or adjusting camera etc. 
 
So I tested again, shut down pc for several hours. Returned and just like before. Got about 5 ish hours of proper performance before the issues occured.
 
Temps were not a factor, monitored them through the entire test. 
 
This is such a peculiar issue, though I really do hope to solve it. Thanks so much for your continued support

Doubtful my power supply is still under warrently. Its nearly 6 years old. It is a Corsair 850w modular. 

what temps u have?? (IDLE and stressed) ur PSU is too old. maybe this is the problem. 
 
pls if u can post ur temps (all cpu,gpu,mb,ram)
 
and if u can try ur pc with another PSU.

defective hardware means total system shut down. so its not. dont think is power supply either. can be run out of memory.