The ASIC was advertised to get 81 TH/s, but really never performed above 79 TH/s when I first got it. For months it performed in the 76-78 range and now is around 73.6 TH/s and decreasing with each passing day.
I think most asic miners are rated at +-5% so 3-4TH missing on an 81TH miner is pretty normal, what's not normal is the diminishing hash rate, this is a very strange problem, I believe your miner is auto-tuning itself and for some reason, it doesn't see the miner at a stable state with the current hashrate so it tunes the frequency to a lower figure just to keep the miner stable.
I don't own a similar gear but speaking from experience with other miners, the tuning part can go wrong at times, so try to flash a different firmware just to make sure the firmware isn't falling at tunning the hash boards.