hp laserjet 2100 paper feed issue

[wipperator]

haven't used this printer in a good while. when starting a print job in the manual tray it will print the first page OK but is continuosly feeding paper in. It's not dragging in paper like the paper is damp or something, it is feeding it in like it wants more paper. This ends with a stopped printer.
Is the some kind of paper sensor that tells the printer that paper is flowing along the right path and to stop trying to feed more paper?