I had an issue where sometimes the computer would hang at startup, during POST, with either a code
99 on the motherboard's debug LED (Asrock Z77 extreme4). If it was the former (code
67), the screen would not display anything, while the latter would display some info on the UEFI splash screen until the code
99 hang. Resetting the PC (sometimes two or three times were necessary) would get it to boot properly. The hangs always occurred the first couple of times after leaving the computer off (shut down) for a while, usually during reboots, and never after leaving the computer switched off at the power supply for a while; also never after resuming from standby mode (S3). The hangs occurred regardless of whether or not hard drives/SSDs or USB devices were attached.
I suspected a faulty power supply, so I replaced the Corsair RM650W with an EVGA 650W PSU. Same problem.
I then suspected a faulty motherboard, so I replaced the motherboard, CPU and RAM. The new motherboard is an Asus X99 Deluxe. Using the original (Corsair) PSU, I get intermittent hangs (though less frequent than before), with the same code
67 displayed on the debug LED, always fixed by a hard reset as before.
I am now suspicious of the graphics card, which is an Asus GTX 970. Once, this morning, I booted up the PC and it successfully went through POST, but with visual artifacts apparent on the top half of the screen until I rebooted.
However, the code
67 hang occurred on the previous motherboard regardless of whether or not the graphics card was attached. I don't have another graphics card handy and the newer motherboard doesn't have integrated graphics.
Does a graphics card problem sound likely, or is there another possibility which I have missed?