Probably a hardware issue, Bitmain's quality has been going downhill ever since the S3, so I'm not too surprised by this. If we're lucky, it'll be fixed in the next firmware update by bitmain; I don't trust them enough to flash my hardware, but if it works, it works.
Just sent the board out tonight to Bitmain Warranty in Denver.
Including the insurance for $450USD total is $20.10 using USPS Flat-rate medium shipping box.
Beats the hell outa sending it to Hong Kong. A s7 board I sent there on Feb 16 was something like $95 but the board was DOA so for warranty repair/replace, had to be done. btw: got the replacement s7 board 1 week later, runs perfect.
Anywho, Bitmain Warranty quoted the repair for s9 boards with similar issues as typically $85-130