Check you reject percentage. Also make sure you are not just looking at the 5 min hashrate because that may have a huge variance. Look at the "Round accepted speed MH/s" and also look at the chart.
I was looking at my Round Accepted speed. According to BAMT, my reject % is 2.1%, which I'm fairly happy with. I guess I'll just keep monitoring what my miners are saying compared to NH, and if I can't get something acceptable, I'll move on.
Thanks.