RX 580 Nitro+ hashrate issues

[Pedro1x]

Hi,
I have 6 x RX580 Nitro+ 8gb sapphire, I modded the bios and clocked the memory at 2250 and core at 1150.
All 6 are at 28 MH/s , but every few seconds I see at least one of the GPUs go down to 3-15 MH/s , then goes back up, and another GPU does the same. All GPUs go to lower hashrate at some point.
The temp is high 77-80C on some. Is it a heat issue? or is it a clocking issue? or something else?
Any idea will be very appreciated.
Thanks gents

[Vann]

Sounds like the cards are throttling from the heat. I target to stay at less than 72C. Try putting a box fan on the cards so you have better circulation without having to turn up the fan speed on the cards too much. Also check for memory errors with HWinfo. I've never seen a bios modded RX 570/580 that can overclock to 2250 MHz on the memory without producing tons of memory errors. On my RX Nitro Pulse 580 4 GB and Nitro+ 570 4 GB cards with Hynix memory I use a a -96 mV undervolt, 1150 MHz core clock and 2075 MHz memory clock and get 28.8 ETH hash, 735 MH/s on DCR with the RX 570 and 925 MH/s on DCR with the RX 580 dual mining speed.

[Pedro1x]

Thank you, I will check the temperature.