Moto krzr k1 problem

I got this motorola KRZR K1 from AT&T about 4 months back. it has this weird problem, like every 1 month or so it suddenly becomes unresponsive, and if some1 calls me my phone doesnt ring, and if i try calling some1 the call doesnt go through. i can access the Menu items but if i try to start my camera , i get the message: "Busy, Try Again Later". then if i power off and on the phone everything becomes normal. I have never opened the phone or even never taken out the battery after it was installed at the store. I did the master reset also (as per motorola tech support), but after that it again happened today.
could anyone plis let me know where is the prob and whats the remedy?