And here I thought everything was rock solid -,-

I need advise, if you have dealt with this.

5 rx 480, flashed bios 1750 to 2000 memory straps, the rest is stock. 1000W Corsair gold PSU, 90~100W per card, 1180 core clock, 2200 memory clock, undervolt -50.
Had stable 30+ Mh/s for each card (2 G1 Gigabytes, 3 Sapphire Nitro (switched into position to flash another bios)), total 150+ Mh/s. Used Msi AB. Temps 70-78.

Yesterday hash dropped to 140, like no OC. And even drops for 2 cards to 20-14-7 Mh/s. Had boot problem too, so reinstalled Windows 10 and AMD 16.9.2 drivers.
Now 140 Mh/s stable. Msi AB does not increase hashrate.

Where to dig?