I have the same issue as Coyn on 2x RX480, since upgrading from 9.0 to 9.1: Siacoin mining speed doubles, but Ethereum mining speed is very unstable and often drops. The cards do not appear to be throttling, temperatur is normal and there is sufficient power.

Adjusting dcri does not help, even with low numbers Ethereum mining speed is unstable. Currently I use -asm 0 to fix it, but of course that causes much lower Siacoin mining speed.