A couple of things tend to stand out to me.
First off, registry cleaners/optimizers/whatever should be avoided like the plague. They have a tendency to cause more problems than they ever resolve or even make batter. You say you didn't use them until AFTER, but I've often found that people who use these programs often have a number of other questionable programs installed. At this point, you may have caused damage to the OS which will require reinstalling. Only time will tell.
Second, 85C is WAY too hot. If it goes above 65C, you should start rethinking your cooling solution.
Third, I agree with the idea to break the system down to it's bare essentials and test components individually. I would also add running a memory test to the mix. I also tend to question whether 650W is sufficient for everything you have in that system. Right now however, I'd be inclined to think that the system simply gets way too hot if you're seeing temps in the 85C range. A quick test for that one is to simply remove the side of the case and see if the problem lessens and/or disappears. If not, then you already have it open to start removing parts you can do without temporarily.
OS: Microsoft Windows 7 Home Premium 64-bit
Case: Antec Performance One P180 Silver ATX Mid Tower
PSU: OCZ Z-Series 650W ATX12V2.2/EPS12V2.91 80 Plus Silver
Mobo: ASUS P6X58D Premium LGA 1366 Intel X58 SATA 6Gb/s USB 3.0 ATX
CPU: Intel Core i7-920 2.66GHz LGA 1366 130W Quad-Core
GPU: XFX Radeon HD 5850 XXX Edition 1GB 256-bit GDDR5 PCI-E 2.0 x16
Mem: OCZ Gold 6GB(3x2GB) 240-Pin DDR3 SDRAM 1600 (PC3 12800)
Fan: ARCTIC COOLING Freezer 7 Pro Rev.2 92mm Heat sink/fan
(4) 120mm Antec TriCool case fans
HDD: OCZ Vertex Series 30GB SATA II MLC SSD (boot)
Seagate Barracuda 7200.12 250GB SATA 3.0Gb/s (storage)
OD: SAMSUNG Black Blu-ray Combo Drive
About a month ago, I built this computer. I made certain to research the compatibility of all the components and check that they will meet the minimum requirements I have for the system (PC gaming). But whenever I start a game, within 1 minute of being in the game (not including pre-rendered video), the audio and video will stutter then the screen will go black and the audio will continue to stutter until reboot. If I open Task Manager and close the game before it goes to a black screen, I do not need to reboot and the computer continues to function normally. This problem occurs across 2 different games (Crysis and BioShock 2) and occurs regardless of the graphics settings (High versus Low).
I have uninstalled and reinstalled the drivers for the video card and all drivers included with the motherboard. I have searched for the latest driver updates. I have tried running the games with the latest video card driver and a previous version. I have reinstalled Windows 7 clean and then reinstalled all the drivers. Despite this, the problem persisted. On another suggestion, I ran the installer for Direct X 9.0c, but I am not certain if that mattered as Windows 7 came with DX11. I also downloaded and ran 3 separate Windows Registry repair programs (Windows Registry Repair 2.0, CCleaner, Advanced System Optimizer 3.0) and after all that the problem still persisted.
I checked all the wiring. Everything is plugged in correctly (8-pin and 24-pin motherboard sockets, 2 6-pin on video card, etc.) I made certain that the memory modules and video card are seated correctly. I reseated the CPU heat sink and reapplied thermal paste.
I tested the CPU temperature, running Prime95 for 1 hour with RealTemp and, under full load, the CPU never exceeds 85C on any core. I ran Memtest86x and after 3 passes, none of the memory modules returned any errors. I ran 3D-Mark Vantage Pro and both the CPU and GPU completed all the tests with a 3D-Mark score of H11111. I am a little confused how the computer can run a 3D processing test successfully, yet fail every time I try to run a game.
I have emailed the video card manufacturer (XFX), the motherboard manufacturer (ASUS) and PSU manufacturer (OCZ). After a conversation with XFX, I RMA'd the video card thinking that would solve the problem but it did not. The response from ASUS was that I should breadboard the computer and test each component individually to see if an error occurs. If that fails to isolate the bad component, they suggested I RMA the motherboard. OCZ suggested that my PSU is functioning correctly because I am able to pass the 3D Mark tests. I wanted to get a second opinion about what to do, so I thought that I would ask here. If I breadboard my computer and still cannot find the problem, could it be the motherboard? And is there any way I could test the motherboard? I would like to aviod RMA'ing another component without being certain it is the cause of the problem. Any advice would be greatly appreciated.