Boot issue

Hello Experts!
Would appreciate any advice on a system that will no longer start. (system is about 7 years old; Phanteks case; X99 OC Formula; i7 5930K; 4x 8GB; 2x EVGA GTX970; Samsung 960 M.2 boot; various other SSD, HDD, cooling).

Disconnected all the drives save for the M.2. When I press Power on the MB, the 2 (out of 5) CPU cooling fans start, all 4 Post Status Checker LED status lights flash briefly, and then evertything stops. All in about 1 second. Then in about a second, it trys again (and again, and again). Sounds like a clicking sound eminating from the PSU.

I have done what I can: reseated the CPU (plus new thermal paste), memory (tried 1, 2, 3 or 4 modules), video cards (tried 0, 1, or 2). Removed and reinstalled all cables. Tested the PSU (all voltages OK; plus it is new). Cleared CMOS. New battery. Tried BIOS A and B. Moved the M.2.    

Of note, earlier today, for the first time in weeks, everything started (finally fixed?!!). Dr Debug showed no codes. However, one of the four PSC status lights stayed on (the second one nearest to the + and - switches. VGA? Possibly - since I did not have the video cards installed at the time.) However, this was a one time event, and it would not reboot after I powered down (did not change configuration). And tried again and again with different configs.

Has anyone seen this? MB going? CPU issue? Any thoughts on next steps? Would rather not hand it over to a repair shop, but I do not know how to determine if this is a MB, CPU, other issue.    

Thank you for reading this long email! I appreciate any advice!