[wake266]

recently my macbook started having problems starting up. I would begin to load and then a low poping noise as it shut down, Some times it would get all the way to the grey screen and make the start up tone, and on rare occasions it would load up and ask for my password before randomly starting up.
i took it to the apple store and he did some combinations on the keyboard while it was loading and told me he reset the power manager and the p-ram. so it looked like it was working. i got it in the car and tryed to start it. didnt work. so for like a month it didnt work. yesterday it worked flawlessly and today it started to not start again. whats up?

[mfbernstein]

What sort of popping sound? Acting up in what ways after booting? Have you run the Apple Hardware Test diagnostics on the machine (hold down 'D' on startup)? Sounds like a flaky motherboard (or possibly, though less likely, hard drive), but you'd have to describe the symptoms more specifically.

[wake266]

the same poping noise you hear if you hold down the power button. If it gets all the way loaded it works flawlessly until it just, well, shuts off

[mfbernstein]

Right. That's usually the sound of the various electronics inside as the electrical current is cut. Definitely faulty hardware. Could be a malfunctioning temperature sensor (causing the machine to thinks its overheating and shutdown) but more likely it's a bad motherboard. I think sending it back to Apple's your only option. You've got all the relevant firmware updates installed, right? It probably wouldn't hurt to reset the SMC one more time.