Ramp up voltage on that card a bit up ... see what will happen, it should work normal with -i 5
Also, take off the extender cables, clean contacts on both card and small board that goes in motherboard. Re-test.
and ... last but not least - you mentioned "modded" 280x ? Which way it was modded ? You flashed some modded bios,
that you found on this forum (bad idea), or you manually modded lower straps into higher ones ?
also, what is the card manufacturer / exact model ?
Alex
If you read next my posts, you will see that i found problem - it was in ttli option. It was bug in 12.1 version - when miner must to low intensity on achieving ttli temperature it causes GPU to hang and restarting.
In 12.2 all working normal as must.
P.S. I don't use modedd bios maked somebody. I maked it myself using my original ASUS Radeon R9 280X DirectCU II TOP (R9280X-DC2T-3GD5) bios.