Will write here, as this thread was the first in google search :]
For Satan sakes - check your AC adapter! When I got this problem, I read over interwebs, asked my friends and friends-of-friends and majority answers were - wither power contact on laptop is broken or motherboard issue. Repair stores, as "funny" as it sounds, were truly consistent on motherboard issue.
BUT
That was wiring in adapter's cord. Even if mechanic checks your adapter, it could SEEM fine [in my case that was it] but don't trust him - just plug in another, known good AC.
I am not saying that this is the only true solution, but well worth to check - will save you a lot of money!
\m/