Who can say - why sometimes speed 1 of cards drops from 31M to 25M untill restarting miner?
Overclock reset. It happens sometimes don't know why though.
I have these hash drops also but it comes back in couple of minutes. Sometime OverdriveNtool loses OC setting for one card usually, just down OC
I use PhoenixMiner for OC (AMD) and MSI Afterburner for Nvidia. Have the drops and they always recover to max hash in couple of minutes. Never required to restart or anything. One of the "why" PhoenixMiner.