where did you get the replacement fan from? I do not think I would disable the cpu fan check on startup neither because I do not think it takes a very long time to burn up the cpu. It is possible that is releated to the mb. I built a pc a month ago and noticed that the cpu fan was not spinning as fast as it should. I went in the bios and found out that is was spinning at about 25% of what is should have been. I contacted AMD and they sent me a new fan/heatsink and the new one worked fine.