That's a really interesting issue, definitely not a congestion thing though. Something is introducing a lot of noise on the line to cause it drop and resync. Is there something like an electric fence controller that comes on, or some other machinery that fires up at about the time the problem?
How do you get it to resync back at the higher rate, does it need a modem power cycle, or is it coming back by itself?
Highly likely it's a street lamp or such, heavy load turning on and changing the RF conditions...