The TPS is basically a rheostat (think volume control on your radio). Over time (usually 15-25k miles) the TPS develops a flat spot right around the setting that equates to 2000 rpm (~55 mph in top gear). Some a little higher others a little lower. That's why it "hangs" at those revs. When you kill the engine, the electrical charge holding the armature to the flat spot releases and the armature is allowed to rotate back to the lower position.