Are you sure that your AMD Phenom 64 X4 9750 2.4GHz is actually at 12C
? Try using other temperature monitoring programs like Core Temp, Real Temp and Speedfan, and see what they report, it might be getting too hot, especially if you are using the stock cooler. Just so you know, the original Phenom 64s run pretty hot, and do not overclock well - 3.0Ghz would be considered to be a very good overclock.
As for overclocking, here is a good guide to get you started. Just to note, CPU-based overclocking crashes are normally either due to heat or because not enough voltage is supplied.http://www.ocia.net/articles/k10overclock/page1.shtml