Constant hard reset needed every time laptop turnt on

Hi all

I'm repairing a hp g62-a30sa, and it continuously hangs on a black screen as soon you turn it on, no logo, no post messages, power is on Wi-Fi light is orange.

I've stripped it down completely, cleared the cpu, checked the heatsink, re applied thermal grease, checked ram, checked the hard drive, both ok, updated the bios to the latest version. I've run bios hard drive test and memory test, both are fine.

The only thing that will get the laptop to boot is a hard reset and it is the same each time I close it down, it just won't boot without the reset. I'm guessing it's something between bios and the software, as the hard reset obviously clears it from the ram. Anyone have any ideas on what could cause it?

Thanks guys.