Question PC keeps restarting before boot

[cowoflight]

Hello everyone - I built my own PC 5 years ago and it's run without any problems since then. Specs are:
Asus Z97-PRO WI-FI AC ATX Motherboard
Intel Core i7 i7-4790K
Cooler Master Hyper 212
2x8GB Corsair LP DDR 1600 Mhz
Corsair RM750 Gold PSU
Samsung EVO SSD - 2x120GB 1x240GB
WD 3TB Red
MSI 970

I haven't made any recent hardware changes to the system and haven't changed any BIOS settings. The PC worked fine without any errors but the next time I tried to turn it on a week or so later, it restarts itself after a short amount of time. The first time it starts I hear a single BIOS beep and it shows the ASUS BIOS screen but then it restarts. It feels like it takes longer to get to the beep too. Then it will enter a loop of restarting but doing so before the BIOS beep. If left unplugged for a while then it will BIOS beep the first time again. The 3 case fans and the Hyper 212 fan all run when the system is turned on.

Steps I've taken so far:

1. replaced the PSU with a new Corsair RM750 Gold & reseated all power leads
2. removed graphic card and attempted to start
3. rotated RAM sticks and tried with each stick on it's own in the primary slot
4. removed the Hyper 212, cleaned off old thermal paste, reapplied new thermal paste (from new tube, not the 5 year old one) and reattached Hyper 212
5. removed & replaced the battery on the MB. When doing so I held the power button for 30 seconds too.
6. removed the power button and reset button leads from the MB and started the system from the MB power button

I honestly don't know what to try next! Because the system stays powered longer on the first attempt I thought it might be an overheating problem but I attempted to fix that already.

Any thoughts would be appreciated

 

[MJS WARLORD]

Are you sure all your fans are running , my pc is to the left of my desk so wife does not catch it with wheelchair when moving to printer , so I did not see that my side fan had stopped and the custom builder had put in an extra system check screen so that it would not go to desktop if it detected a problem. With warm air passing over the wire the fan plug had moved just enough to stop it making proper contact.

Take a look in reliability log it might show a red x to indicate a problem , if it does , google the error code.
Next run hardware reliability monitor.

As you have been taking items out i think you would know if they were dusty !

 

[cowoflight]

Hello - thanks for your reply. All fans are running and I checked the dust filter on the front of the case to make sure it wasn't clogged stopping air flow.

Is there a way to use Reliability Monitor on a different machine? I can't get to Windows to run it on my desktop but do have a laptop. I've just opened it now on the laptop but can't see an obvious way to change system