I had a similar issue with a 280x toxic, clocked at 1150/1600. I tried lowering clocks, raising voltage but the card was running wild to up to 320 h/s and a lot of rejects and errors. Lowering intensity to 4 solved it. For some reason, cards even within the same chipset behave differently.
One of my pair of R9 280x (both are the SAME model of Gigabyte Windforce card) gets very wierd unstable and unhappy at anything over -i 4 but runs reliably at that intensity.
The other one will run at anything up to -i 8 but seems to give the highest reliable hashrate results at -i 6.
My RX 470s were happy at -i 8 on v9.1 but got very crashy on v9.2 at same settings, dropped them to -i 7 and they're a lot happier - then moved them to v10 and they're still happiest at -i 7.
(definitely not an inadaquate PS - that machine is running on a Seasonic X-1250)