I would try a different pool , if ( as you mentioned) that you tried a working psu, resetting and all the other normal procedures, then it's only one out of two possibilities, connection problem ( pool , router firewall not allowing the miner to connect to pool etc,) or simply the control board has gone bad.
I am currently using slushpool and the interesting thing is this is only happening to my S9 machines, and they stopped restarting however they have a very high stale rate, while at the same time my s11 machines have no problem
What is a good pool you suggest other than slushpool?