Gigabit,
The second screen capture you supplied gives an average hash rate of 368 GH/s (and that's including the anomalous 333 GH/s).
Where did you get 313 GH/s? None of the values in the table are anywhere near that low.
NEW MATH! What is this socery?!
My mining has always been incredibly consistent with BitMinter as long as my miners were running - regardless of pool size and regardless of difficulty.
Gigabit has it been getting warmer where you're located? I noticed my miners slowed down a bit as the ambient temperatures rose and I lost some speed as a result. Just a guess but given the time of year it's certainly possible.