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.
The problem with 480 is that it has just one BIOS, if there is anything wrong, you might brick the GPU.