Random black screen hangs during POST on computer restart.

Hi everyone.This is a long post so bear with me please. Thanks. My problem is somewhat weird. I've searched and searched for a solution but none was like mine. So sometimes (actually most times) after i do the smallest change like stop/start a service , or set the number of drives in daemon tools and such, upon reboot, my system just hangs on a black screen during POST. So it doesn't even get through post. Most of the times i have to manually restart the system but it doesn't always boot after the first restart, sometimes it requires several manual restarts. It's like the computer is in sleep/hibernate mode or something.This all happened after i installed my new video card (a GTX 260 Core 216) which supposedly eats up a lot of power. Of course i also installed a new 450Watt power supply and made sure it has 37 Ampers on the +12v rail as the producer recommends and further proceeded with a fresh Windows 7 install. Now i'm guessing this is either a boot sector problem , since this only happens when system changes occur (except new software being installed like games since this does not affect startup programs) OR as i mentioned earlier , maybe the videocard doesn't get enough power(?). Thanks for reading and hopefully someone might have a solution for my problem.

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