What is wrong with my hardware?

So a few months ago I built a custom PC with Vista 64 bit installed. Everything was going great, no problems, and then I decided to overclock my CPU. I increased from 2.40 to 3.0 ghz, unlinked with the RAM and without increasing voltage. I had a Tuniq Tower attached as CPU cooling so it should have been enough to cool the CPU at the new clock speed.

Everything went fine for a few days until my system started crashing. I ignored it at first but then the frequency increased. It would even crash once every hour or so. I reverted my CPU clock speed back to 2.40 and the crashes ceased to occur. However, there seems to be a major impact on certain performance aspects of my system. In any in-game cutscene of any game of any time period of any genre, when the camera pans around, it would stutter every second or two. Scrolling/Panning around the map in RTS games such as Empire: Total War would cause severe choppiness and stuttering. Even streaming certain videos from the internet (fully loaded) would cause choppiness.

Is there something wrong? I ran memtest 86+ to test my RAM but it ran through 16 tests and it all passed without any errors. I ran stress tests on both my GPU and CPU but everything seemed fine. I even reformatted my system with a new OS but the problems still persist. Could it be the MOBO? I want to call in a warranty but how can I prove something's broken? Would overclocking void warranties on MOBOs? Any help would be greatly appreciated.

System Specs: Antec TruPower Quattro 1000watt PSU, an EVGA 9800GX2 stock clock GPU, Q6600 @2.40 stock speed, 4 X 1gb ram @1333 (4gb), 790i ultra MOBO, Windows XP 32 bit SP3

^ Thanks, I have increased voltages slightly before but it still crashed. My system still crashes now, just less frequently (once every few days or so).

I won't tell them I O/C. However, is there a way for them to know.
Anyway what's wrong with my system right now? Which component is actually damaged?