I see,if at stock then it can be explaing but stock voltage is 1.13+ vddc,undervolting to 0.9 will decrease power consumption dramatically so u should not have more problems when undervolted...
Another problem is sometimes on a reboot wattman settings will reset to default if youre in a hurry or you have the miner set to auto start on reboot you cant always be guaranteed your settings will persist.
We really need a solid bios mod for these cards like the R9 Nanos. I have six nano rigs that run for weeks with zero issues heavily undervolted , the Rx 480 needs this since the software is so flaky atm.