it was shitty Gigabyte thermal interfaces. Never buy Gigabyte.
Did someone else encounter the problem is melting thermal pads on Gigabyte cards? Gigabyte RX570/RX580 Aorus in particular.
In my mix of cards I've only one Gigabyte cards, an RX480, and it has not been melting, but I've two XFX RX580's and they all have the melting thermal pad problem.
Some people have had this problem with Sapphire RX480.
Pic of my 'oily' RX580 in this thread (post #1095).
https://bitcointalk.org/index.php?topic=1874892.1080