Fresh built PC having problems

[Lee_31]

Hello.
I built new PC a weeks ago and it having few problems. Please help...

First, PC turns off all of sudden with fan still rolling on and off repeatedly then turns on by itself.

Also, the other day, I tried to shutdown like usual, screen was turned off but power light was still on with an active fan again. So I was forced to shut it off.

I personally think it's either motherboard or power problem but I not too sure how to fix.

Please advise me. Thank you in advance.


edit:

This is my system spec as it is... I apologize for not including it earlier.

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i7-4790 3.6GHz Quad-Core Processor ($294.99 @ SuperBiiz)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($29.49 @ Amazon)
Motherboard: MSI B85-G41 PC Mate ATX LGA1150 Motherboard
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory ($32.99 @ NCIX US)
Storage: Samsung 850 EVO-Series 120GB 2.5" Solid State Drive ($63.99 @ NCIX US)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($47.49 @ OutletPC)
Video Card: Zotac GeForce GTX 750 Ti 2GB Video Card ($96.88 @ OutletPC)
Case: NZXT S340 (White) ATX Mid Tower Case ($63.99 @ SuperBiiz)
Power Supply: EVGA 500W 80+ Bronze Certified ATX Power Supply ($39.99 @ NCIX US)
Total: $669.81
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-05-31 23:12 EDT-0400

 

[Lee_31]

Have you checked that either your motherboard power pins (the ones that you plug the case power switch into) are not shorted, and the case switch not held down

Yes, I thought the power cables were fixed little bit too tight so I loosened them once.
And pins are in the right places too...