I don't think MTS is throttling your connection. After some other ISPs (not MTS) were caught throttling, the CRTC developed rules that ISPs had to make their throttling policy public.
My guess is you are having intermittent line problems that cause the modem to back off until it can operate stably. The problems could be caused by moisture even if you have overhead lines.
I would request MTS do a line test and swap the line if there's a spare pair available.