1. spend 150 bucks and get a decent psu.
2. dont push the cards...900/300 will get you just as many shares as 940/300. and definitely don't overvolt.
" if you buy and car and floor it, how long do you think it will last. put the car on cruise control."
Don't over-volt, Your one crazy man not getting the most out of your hardware, theres at LEAST a 15% overhead on EVERY ELECTRONIC GOODS SOLD. i run 5850's at 965/300 @1.193V (they do 1000/300) and they come 725/1000 @ 1.088V stock. multiply that by 8 as i have 8 of them and its like having 11 cards, not 8. Up to you but as long as GPU is below 85, 90 max, VRM's below 110 you wont have an issue. At least for 3 years which if your still using them then you'll be wasteing alot of power per m/hash you'll be getting. Then again, new architechre of AMD will ruin it for miners so its looking like FPGA is the way to go.
Back on note, Just take the system apart and back together again, Just incase its a silly loose connection. Sounds stupid but this as helped me no end in the past. Never actually finding the cause of the problem though. Your OC's might be high, If the cards were new and you pushed them without 'wearing them in' they may not be able hold at what they first did. Try a slight lower clock.
Is it warmer where you are? Extra heat means extra stress. Why i put mine to 965 and not 1000 @1.2V, just allows for fluctuations in ambient temperture more.
Failing that, ballache but try running each card individually. Check the fans are ok on them, i had 2 XFX's sease up on me. Cheap parts. So many things its just a matter of fiddling and narrowing down the search
I'd pin it on your PSU though, so try the 5870 alone and see how it goes. THat will be a good indicator of PSU problem. Corair AX series i cant reccomend enough, Pricey yes, but you know its not your PSU then. there 1200W can do 1550 on 12V single rail with little fluctuation