also, I'm using the stock HSF. What am I doing wrong??
I think you might have answered your question before you asked it.
Are overclocking the CPU? That could cause high heat, especially if you raised the voltage. Try going into BIOS and setting the CPU fan to run at 100% all the time, instead of smart fan or whatever they call it. Also, the LGA775 heatsink mounting method is a pain in the neck, you might have one of the four posts not engaged properly. Maybe try removing and reseating the HSF.