I'm at a loss as to what else could possibly be causing this, but I've no experience of mobo issues so unsure whether it is actually the problem.

Board is a Z77-D3H, I used to power it up and down via sleep (W7 64bit) with no problems, then out of the blue it started to get random whether or not it would recover from sleep properly, it would either work faultlessly or I'd get the error beeps and it would restart before actually getting anywhere and give me the whole "Windows did not shut down correctly" spiel.

It then slowly progressed to where I am now where not only will it not restart from sleep but it won't restart from a total shut down either, the only way I can get it to boot is by turning off the PSU for about 10-15 seconds and then it boots as normal with no errors.

Once booted everything works 100% flawlessly.

Things I've tried- replaced the PSU, fresh install of windows, resetting CMOS, flashing BIOS, unplugging all USB devices and network cables. None of this has made any difference, so I'm left with thinking this is a motherboard issue, it can't be a BIOS setting I'm unaware of because I've flashed and reset it so many times now.

This is a pain in the neck now because normally the PC case is hidden out of the way, hence me using sleep a lot, at the minute I've got to have the case dragged out into the room so I can reset the PSU from the case switch before every boot.

Rest of spec, if it helps, is:

Be Quiet Pure Power 630W PSU
Gigabyte GA-Z77-D3H mobo
Core i3 2100
16GB Patriot Viper Xtreme DDR3 PC3-12800C8 1600MHz
64GB SSD for OS
2TB HDD for storage
GFX VTX3D 7870 Black
Dell 24" LED 1900x1200 connected via DVI
W7 64bit