Urrr.. You might just check the maximum speed set for your modem in Windows.
Start/Settings/Control Panel/Modems/Properties (for your modem)/General
Check the maximum speed setting - if it's too low, jack it up; if its already at 115200, try bringing it down to 57600
Just a thought!
BTW my connect speeds differ from 48000 (max) to 32000 (min) depending on I dunno what (worse when windy and raining though).