Rigs restarting after around 1 hour

[KingPush]

I have two rigs with 4 XFX R9 270s each and they both restart themselves after around an hour when trying to run anything above 380 Kh/s. I have 2 other rigs with ASUS R9 270s and they run without restarting at 450 Kh/s.

I just tried leaving two cards running last night and one of them read "DEAD" and the other was still going. I've tried reinstalling drivers, didn't help. I was thinking maybe it's a lack of system RAM, but then why would 2 cards not run? Have 4GB. And the ASUS cards run so I doubt it's that. What could be the problem?

[frankenmint]

need stronger PSUs - this is the case usually 8-9 times out of 10.  What are they?  Also are you using powered risers? Its recommended to use  pcie connections for your graphics cards themselves to provide that extra umph of power.  I reccomend at least 1300 Watts or higher per 4 gpus.

[KingPush]

How come I can't even run two of them properly at anything higher than 380 Kh/s then though? And why / how would the power supply power them for an hour then just stop? Seems weird. I'm running them on HX850 80 Plus Golds and using powered risers on all cards

And you have to remember, I have two rigs with all ASUS R9 270s running fine at 450 Kh/s with the same PSUs. Unless the XFX cards for some reason are super inefficient, it seems weird