Screen randomly goes black, sometimes monitor say no signal

Ok heres the deal. I buil my computer 7 months ago, and this is the problem.

When I do anything, sometimes as little as scrolling the internet, or even talking in ventrilo, my screen goes black. Sometimes it just goes black as in no color cant see anything, sometimes the monitor completely says no input signal.

THINGS IVE TRIED:
Reformatting
Scanning with every virus/malware program i can find
CCleaner
Driver cleaner, reinstall drivers, tried 3 diff versions
Checked connections in case, cleaned case very well
Nothing is overheating. Nothing is overclocked.


SPECS:
gigabyte p35 motherboard
sapphire 4870 ddr5 512mb
2 gigs mushkin ram(3 gigs tomorrow)
550w Corsair VX power supply
samsung drive
WD 595 7200 RPM HDD
Huge, roomy full size case.

If anyone can provide insight it will be much appreciated.This has happened with all 3 drivers ive tried, new and old, and it just started happening within the week. Sometimes it happens sometimes it doesnt. There seems to be no noticable trigger of it happening. IT NEVER HAPPENS WHILE IDLE!
This leads me to think its the PSU. What to do, what are your oppinions?Thanks a ton for your time.