Epoch changes affected DAG size which caused us to see performance drops. This was because of the way AMD drivers address graphics memory, it did not affect performance in games etc, but crippled Ethash mining. With the blockchain drivers AMD fixed this.
In newer drives, you have to go to AMD settings - gaming - global settings and switch to compute mode from the default gaming mode. This fixes hashing performance.
And what to do with Nvidia to avoid epoch-connected hashrate issues? No "compute" switch for 9xx Nvidia cards :-(
BTW not even half videomemory was used in either case.