Bufius,
Are you sure the card is OK? I've got Sapphire R9 290s (non-X), the standard reference design (with the hairdryer-like noisy fan).
With the engine @ 947 and mem @ 1500 and Intensity 18 my cards get 76C. Fan runs at 75%.
Higher intensities seem to crash the card at some point, as does a higher clock. Just FYI. I find 93C at 700 wit I14 a bit suspicious.
Edit: Ok, temp was before you fixed the paste problem, but still the crashing is odd. I *can* run the cards at 1000 clock, but they crash after hours/days then. At 85C btw.
I am sure the card is not really Ok, at least the custom cooler is not the best as it seems.
And the stock BIOS has really low voltages, this is the main cause of the voltage based crashes.
But i will apply again (even thinner) new compound. Perhaps i applied too much.
But the conclusion is the same: the mining application kicks 100% constant load to the card and this is much more than e.g. normal games and even more than furmark (i could not believer this before my own testings).