System: GA-Z170-HD3 motherboard, i7-6700K CPU, 4x8G RAM
This box has worked perfectly for a year and a half, until today I took out the video card (Zotac GTX 970) to investigate one of its fans that had become noisy. I didn't touch anything besides removing and replacing the video card, and reseating a USB header cable. On booting, it now, after an unpredictable length of time, makes one very short beep and then five longer beeps, then powers off and shortly thereafter boots again, showing nothing on the screen. This occurs with the video card in, or with it out and connecting the display to one of the integrated graphics outputs.
The beep codes for this motherboard are poorly documented, but some research indicated CPU error and some other research showed people having this problem due to RAM issues. I've tried reseating the CPU (no damage observed on CPU or socket), reseating the RAM, and removing all but 1 or 2 of the DIMMs and changing which ones are left, and nothing changes the problem. What do you think I should look at next?