Hmmm, can only suggest that you double check ALL physical connections inside and outside of the PC. Especially mobo-to-drives ribbon cables, which can become slightly dislodged - so don't just look at them, give each a bit of a push.
Also check RAM and PCI cards are ALL firmly seated in their slots.
Failing that I don't know off-hand what else to suggest.
You say you replaced the CPU 2 weeks ago, is the voltage setting correct (i.e. too high-(e.g. have you overclocked it?)- Check Bios), Does it need a bios update to enable the motherboard to run with the new chip?, or it could be the chip is faulty, and needs to be tested by an engineer or returned to the supplier as faulty Terry