...and in addition to all of JoeMikeB's good input, don't forget that you might have something as simple as a marginal aim-point to the satellite due to a shift in your antenna position for some reason....this too could give you highly varying throughput as the atmospheric conditions change during the course of the day.
I wouldn't give up on Hughes until I had a local technician come out to verify the physical installation first....antenna aim-point, amplifier condition, coax condition, ground distribution equipment condition, etc.
