You should be able to unplug the kickdown switch (underneath the accelerator pedal), to allow you to test drive without it. It sounds like yours is stuck.

Here's what you're looking for:

Kickdown Switch (pictures)

__________________
1982 240D, sold 9/17/2008
1987 300D TurboW124.133 - 603.960, 722.317 - Smoke Silver Metallic / Medium Red (702/177), acquired 8/15/2009
262,715 and counting