This is a custom built computer. components:

amd phenom II X4 955
asus M4A88TD-V EVO/USB3
4gigs (2x2gigs)DDR3 OCZ ram (have switched slots)
antec mid tower 902
HX750corsair PSU (modular)
seagate barracude 7200 rpm 500gig
HD Radeon GPU. (problem exists without this installed)

So, this problem occurs every other power on. If i turn my computer on, the fans start running but the unit does not post. I power it off, power it on again and it posts just fine. It does this *every* time. I cant think what the problem could be since it's not random. Any thoughts?

Is it possible that you've either omitted or wrongly connected a psu to board cable?