Gigablast Isn't

When first I subscribed to Gigablast, my download speeds from the modem ranged around 750-850mbps.  Over the last year, I've noticed it has slowed.  Over the past 90 days or so, its dropped to 350 to 550.

I have contacted tech support four times to learn the problem.  I eventually get cut-off on both chat and speaking over the phone...and nothing gets resolved.  I specifically asked if I'm being 'throttled' but they tell me they don't do that.

There have been no equipment changes and, other than an occasional firmware update for the mesh router, nothing else has been altered here.  Is there a secret handshake, a tip jar or bribe account anybody may know about to get Tech Support to actually help?