Hey,
I have an s9 that hashes great for a few hours (anywhwre from 3 - 8hrs), then the pool I'm at stops receiving. Not just momentarily either, this last time it stopped for 3 hours (didn't check it for awhile...). The gui on the s9 shows it's still hashing away just fine, and it shows all pools alive. Once I reboot the machine, it hashes fine for awhile and then stops. I use 3 mainstream pools (viabtc, slushpool, and Nicehash) and it doesn't hash on any of them for more than a few hours before conking out.
I'm using an APW3++ PSU on 220v, so power shouldn't be an issue.
Any thoughts?
Sounds like network connectivity issues. The miner GUI allows for some very basic troubleshooting. You should take advantage of that the next time you think it drops.
Some other things to check, what's the temp? Are you 110v or 220v?