Is my PSU dying/insufficient?

[xXxREBELOxXx]

Alright so, I upgraded to an RX 580 8GB not long ago and under high load it would cause my entire PC to reboot. That kept happening until finally after playing GTA 5 for about 5 mins it rebooted and my GPU literally blew up. I was thinking because it looked refurbished, thought it was just a bad card. Now I have an RX Vega 64 and it's doing the same thing when I load up the GPU to 100% at stock settings, just causes my PC to shut down and reboot. I have a FX 8350 overclocked to 4.75 Ghz and I know that on it's own uses a lot of power maybe 350-400 watts. I also have a almost 3 year old Corsair CX850M (the old green one that isn't rated as the best), but I thought 850W would be enough. I think that at this point it's the PSU because my R9 380 4GB only pulled 160W max even when overclocked to it's max, and that would never cause a crash. I really don't wanna blow up a $900 card I just bought today so I'm looking for any help. I was also thinking of getting an HX1000i to solve any power issues I might have. I really don't want to spend money when I don't need to or re do my wiring so I'm not gonna jump the gun just yet. Thanks for any help, it's all appreciated.

Link to PSU I'm looking to get: http://www.canadacomputers.com/product_info.php?cPath=33_1238_441&item_id=074572

 

[hang-the-9]

First thing, don't overclock when testing. There is no way to know if the crashes are caused by something being overclocked or not. Set things to stock, make sure temps are OK. If you can test the card in another system with a good power supply do that. If not, and running at stock speeds causes issues, then trying another PSU is a good plan.

 

[xXxREBELOxXx]

My overclocks were stable as I did hours of Prime and gaming (I've found GTA 5 is the best for finding unstable clocks IMO). Once I put in anything except my R9 380 that's where problems came up, which was random reboots under load. So I bit the bullet and went with a EVGA P2 1200W PSU for it's low ripple and to eliminate power as a concern when overclocking or upgrading. Turns out after replacing my PSU the problems went away, it's been about 2 weeks since and haven't had a random reboot yet *knocks on wooden desk*. I read up on those old green CX750w-850w power supplies, they were pretty unreliable after a year or two. New ones are better though.