No display on boot, PC keeps rebooting

It started about a month ago when I was playing graphic intensive games, the PC would switch off then a second later come back on ( and sometimes during boot up after this happened only ) I thought it was a heat issue, and started to clean my fans ( was a fair bit of dust ) but the same thing happened on 5 or so different games. Last Friday week I switched the PC on and there was no display at all. No beeps, no reboots, the PC would power up, fans all spin, and hdd light ticking away, just no display from the HDMI. I tried switching to DVI ( tried both dvi outs ) and still no display on boot, no bios screen nothing, just blank. I tested the TV in another PC ( dell optiplex 270 ) and it worked fine. I figured the graphics card was dead, and put the PC aside until I could afford a new one ( lost my job 3 weeks ago, funds are tight ) but during the week while trying more diagnostics, the PC booted, and the display came on. It went into windows first time, and no problem. I went into catalyst control center, and the temp was idle at 40-43c but I noticed overdrive was on, so I turned it off. After trying to load Star wars the old republic, the PC rebooted, and just kept rebooting, so I switched the PC off, and now its back to no display on boot again.

I went out and bought a cheapo graphics card just to test, but with the new card in, the PC lights would come on for only a half second, same with fan, then go off, then instantly come back on and repeat. Plugged the old card back in and it boots, fan spins, dvd drive kicks in, and HDD ticks away fine, just no display.

I have reseated EVERY cable, tried booting with no ram, one stick at a time, and taken the mobo out for a manual inspection, but I cannot find the problem. I even returned the cheapo gfx card and got a Geforce Gt610 but the exact same thing happens, boot for half a second then off repeated. I tried both cards at the same time, then took the ATI out, and the pc booted with the display working, but constantly rebooted, when I switched the pc off and back on, back to the original problem. Got to try another PSU today, same problem.

its a pc that was custom build about 10 months ago, never had any problems until now.

Specs:

ATI Radeon 5830
4 Gig DDR2 ram
300 gig HDD
700W PSU
Q6600 quad core 2.4 ( never overclocked )
P5k Motherboard
 
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