150kbps is clearly out of spec and unacceptable. While you are shopping for a modem, check if yours has an SNR (signal-noise-ratio) setting. If it does, try changing it up and down a step at a time to see if that helps. You usually have to reboot the modem after each change.
Look for that setting in your choice of a new modem. My older model Billion has it in a hidden page I discovered via http://forums.whirlpool.net.au/archive/1567481. Some modems have it in a regular user menu. Your modem will also report what it “sees” as your copper line quality (the metrics referenced in the forum).
