That is probably why. Also Xfx cards are not the best for overclocking.
But the 5830s are the same, and they have given me no problems
The 5850 puts out significantly more heat at the same clock so it's always been the case that you can't stably overclock them as high as 5830's could be clocked. Based on the clocks you mentioned (900Mhz) and the fact that you're playing with voltage it's almost a sure thing that you cooked the 5850.
Temp readouts are notoriously inaccurate with video cards anyways and I find it pretty hard to believe you never went past 70 degrees just based on your description. Not to mention even if the chip itself is staying cool enough the voltage regulators won't when you're overclocking and overvolting like that. Unless you have serious cooling for all the VRMs and the GPU I wouldn't really recommend overvolting the 5850.
If I were you I'd probably just push it as far as it'd go at stock voltage without crashing. It'll probably be somewhere in the low to mid 800's or so though a lot of people can't even hit 800. Very unlikely to be worth the investment in cooling you'd need to overvolt and overclock the 5850 >900Mhz safely. Check the overclocker forums too, you'll see that very few people hit the kinds of overclocks you're looking for and even if they do who knows how long the cards will last.
Some of the 5850's do have better cooling than the others too, but they tend to be more expensive so if you're looking for value for money it doesn't really make sense to pay that much more to get maybe 30Mhz more.