Aurora R4, random shutdown and restart

Hello Everyone, 

I have an Alien Aurora R4 I bought in 2013 and its been going strong but has been facing issues.

Its been slowly dying. The most recent issues is the most serious.

The issue is that the pc randomly restarts or shutsdown.

Its been happeneing for about 2 months and its gets worse everyday and shuts down more frequently and stops restarting. 

I have been doing some research and it seems like either the motherboard or the PSU power supply is causing the problem. 

I downloaded Dell Support Assist and did all the tests and drivers. Still not fixed. I tried the Diagonostics option and checked all the components and the motherboard check came out good.

So it seems like the Power supply is causing the issue. 

So my questrions is how do I go about this? Should I change my PSU? 

Any idea of how much it will cost or how I can change it?

Any help or suggestion will be greaty appreciated. 

Please feel free to ask any questions. 

PS - I am a 3D Artist so facing this issue is really affecting my work so if anyone has faced this issue before, pls help. Thank you.