I cannot recommend this ZEC miner. Absolutely unstable. I tried it on 6 rigs (all were stable with eth mining), but I have only problems, on some it stopped work in very short time, or hashrate dropped to 0, or it crashed.
If is working hashrate is high, but...
I would recommend - runned it on 16 ethereum rigs ( 280x, 390, 290, 290x cards ) over night and just 2 rigs dropped one card.
What AMD driver version are you using for your 280x? I have 5x 280x and I have one card that always crashes the display driver. Also, are you using stock firmware on your cards from the factory? Undervolting or anything like that? I think my firmware was setup for scrypt way back when. My miners have been off for 2 years and have since created a dual boot ubuntu (for wallet) and Windows 10 to get this thing back mining.