i don't know if anyone else has had this problem before, i just got my macbook back from apple 2 weeks ago (i had it sent in because the logic board failed) but now no matter what i'm doing on the computer, i could just have it sitting there, about 5 minutes after i turn it on it will freeze and i have to hold the power button to turn it off and back on. i tried reseting the pram/nvram, holding the option key, the c key (which caused it to freeze on the gray screen) and the safe boot as well. some of these will extend how long the laptop lasts, but it will still freeze.