PC stars and shuts down, but won't reboot

[Jynnantonix]

Hello everyone, a friend suggested I try get some help from the knowledgeable bunch at hexus , so here I am!

Having a problem with my upgraded rig (specs in sig), which exactly as the title suggests, my pc starts and works absolutely fine. I can play games, run prime, 3dmark etc and haven't experienced a single freeze, stutter or bluescreen. It also will shutdown without a problem, however when I do a reboot, the screen just goes blank and doesn't come back on.

It's not an OS problem, because the same behaviour occurs when I reboot from BIOS without making a change. The only time rebooting works is when I make a change in BIOS and 'save and exit' the reboot process seems a bit different as the fans stop for a second and then come back on, I guess like a 'hard' reboot vs a 'soft' reboot.

The upgrade was from a phenom ii 955 on Asus mobo to my current i5-4670k on a gigabyte Z97X-d3h. Everything worked fine on the old system.

So I tried replacing my new radeon hd7970 with my old HD6950 and this fixed the problem. However the 7970 was working absolutely fine on the old system. So this made me think perhaps this cpu is using more power than the old one, but I checked the charts and actually it consumes less power, so it can't be an overload issue. It would seem a weird PSU problem as the pSU worked absolutely fine on the old system.

So I'm at an absolute loss on this one and any ideas appreciated. Thanks

edit: oops my sig does not seem to be working. system specs as follows:

i5-4670k
gigabyte z87x-d3h
gigabyte radeon 7970 GHz
12GB Corsair Vengeance 9,10,9, 27 @1.5v (tried just using 8GB too)
OCZ MODXSTREAM PRO MODULAR 600W. (bought in 2009)

[Pingu2003]

might be the reset cable shorting

does your motherboard have a reset button on board?

if not try shorting the reset pins with a screw driver and see if that works?