I have an AMD K6-III+ 450 mhz CPU which I bought before XMAS - and I had overclocked it to 600 mhz. Recently, my son mentioned that Windows would crash after 2 hours of heavy gaming...so, I ended up downshifting the speed. At first, I did not think that temperature was the problem since the BIOS did not report very high temperatures (up to 115 F after heavy gaming). But I decided to pull out my plain jane cooler and replace it with an Alpha 6035.
What a difference! I'm able to run it at 616 mhz (112 fsb X 5.5) at 2.1 V core with the BIOS showing the temperature at 104 F after heavy usage. And it is rock solid stable now. I might try reducing the voltage back to its factory 2.0 V setting just to see if it will run OK.
Prior to this time, the fastest speed I could obtain was 600 mhz (100 fsb X 6.0) at 2.2 V...and it was not completely stable after running for a long time - blue screen of death would appear.