I've got a similar issue with my pc. It doesn't post after a cold boot, but works fine after it has been reset. Try switching it on, leave it on for about 30 seconds and then press reset. I've just send my pc back to the shop (again) for repairs. Last time they repaired it by replacing the motherboard with another P5K. It lasted about 2 weeks before the issues returned. (Btw if you visit the Asus or Corsair forums you'll find that there's a lot of people with similar issues with the P5K series)