grifter I had that problem too but I realized the shit windows tells u Is bullsh1t. IT has NOTHING to do with your actual speed.
My modem used to vary alot upon connections (as u said)....now I've made it show me I am connected @ 9,600Bps....sometimes it tells me i'm @ 115,2Kbps...
I use a 56k modem so if that's not bullshit then what is?
WEll I'm not bringing an answer to your question sorry, it was just to say don't rely on that supposed speed windows shows u on top of the 2 flashy PC in your system tray.