I have an issue with one of my S5's that has ran the modified cgminer 4.9 firmware and been overclocked to 375M for around 2 months. (Other is doing very well but underclocked now)
It had numerous hw errors when I checked it (like 50%) and seemed to be not hashing at all a few days ago. It is in a utility closet in the garage, but temps stayed within 50-60c range 95% of time.
I have troubleshooted the problem and right now I am running at 200M stable (with a reboot as I write). As far as I can tell it is not just isolated to one board as both seem to having same issues, when I bump frequency up of more hardware errors,instability,no hashing etc.
So far I have changed power supplies, changed pools, ran stock bitmain firmware, reseated all cables, and looked for noticeable signs of heat damage.
I am thinking of breaking down board and checking thermal paste next, but wanted to get some responses as to the next logical steps... (Will probably have no luck with warranty)
How hot was it showing on temperature? If my math is working that is around 7 percent OC. So does not sound horrible from that side.
Checking thermal paste is not a bad idea if out of warranty. As far as warranty you need to have purchased it direct from bitmain and be within 90 day's of purchase.