Similar to other problems, but not the same

Hello,
So i've been having these problems for as long as i've had my pc. Theseare the symptoms. So i built my pc last year october, got myself a GA-G31M-S2L gigabyte mb, 4 gigs mushkin ram, xfx9800gt 512mb, an e1260(from a friend) a 550W gigabyte superb psu. I currently have 1ide 30gig hd, 1 120gig sata and a 500gig sata. Oh and a lg dvd writer. So my computer has never given me any performance problems during games. But it is a very unstable pc. This is normal startup with my pc: i would turn it on and all the fans would spin up, my gcard fan at 100%, then all would slow down and then it would post and enter booting up screens. This is non startup- turn on pc, all fans spinning, gcard at 100%...and then nothing, no post, no booting up screen,just black monitor and orange light. It would stay like this till i turn it off. At first i lived with it as all i did to remedy the situation was open my case, remove my gcard, reseat my ram, reseat gcard and it would start up fine again. Eventually this wouldnt cut it anymore, and no matter what i did my pc wouldnt post, so then i reinstalled my os(vista 64bt) and it would be ok for a bit, but then start the same old cycle. I did overclock my cpu(e2160 at 2.4Ghz with default voltage as prime95 was stable for 3 hours...) with all precaustions taken, but it ran stabily during games and temps were 40 C at gaming load and 50 C at 100% load. But even when i run it at default settings it does this with my pc. Could i have messed up my cpu? I thought i did, but i removed my gcard and ran onboard and it has started up fine everytime, this lead me to believe it was my gcard, but if my gcard was screwed it wouldnt work at all. And when my pc did work with my gcard in it worked fine. Ran memtest86+ on both sticks, no errors...so if anyone has any clue, help me please!!! Sux not knowing if my pc will boot up from day to day...