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)