I have seen this problem for over two years now, and have a 30 year background in electronics, circuit design, RF, software, and computers - so have quite a bit of understanding about what I have seen, even if Hughes's tools will NOT isolate the problem. It took me literally MONTHS to get to a Hughes engineer with sufficient expertise ("Tier 3") even to understand, and finally ADMIT, the problem.
It is NOT fixed!
In short -- it is this:
Do you have a Motorola HN9000 modem???
(I've been through 3 by now - ALL of them have the problem.)
It is a firmware bug (built-in software) in the HN9000 modem, or data usage handshaking. It will occasionally bill for data usage that is not only NOT being consumed by the customer, but it fact is simply IMPOSSIBLE for the slow satellite downlink even to support if it was operating at a good speed!
If you see a dramatic decrease in system performance, CHECK THE USAGE! You may catch it "in the act." I suspect, but cannot prove (their tests are woefully inadequate) that it is BILLING the customer for packet loss and retries, or some other "housekeeping" that doesn't represent "real data used [downloaded]."
Hughes refuses to let me talk to Tier 3 support without jumping through worthless hoops that I've been through more times now than I care to count. "Bring me the broomstick of the Wicked Witch of the West!" is a good description of the attitude I get every time I see this crop up -- again, and AGAIN.
Let 'em know it TAKES AN EFFORT on their part to fix!