Rainman,
I hate to tell you this but it's about a 95% chance your hard drive is dying. I've had two computers with the exact same symptoms and both of them lost their hard drives completely within a couple dozen power up/power off cycles.
I'd recommend you backup your data the next time you turn it on and wait for the other shoe to drop. Or just stick a new HDD in there before it goes totally tits up.
BeerSlayer is right, it could be a failing controller, bad board, aliens or the Klintons. But most likely your HDD is about to go south on you.
Good luck
Crash.
(A+, CCNA, MCSE +I)