I'm pretty confident about this one...
Diodes on the motor!
There are two diodes soldered onto the motor switch assembly. They tend to corrode, despite the plastic coating. Can you believe it... the motor and it's switches (all the moving parts) don't give a problem - but the solid-state electronics corrode. This would be why the Italians never made PCs. The transistors would go rusty.
So... to fix, you just dig out the green plastic resin that fills the slots on the back of the motor cover. Then unsolder the diode and replace with any generic type (which should cost mere pennies), e.g. 1N4004. Don't overheat the tabs on the motor or they will sink into the plastic and make the job much harder.
You should have it sorted pretty quick, but mark the shaft of the motor (scribe a line across the operating arm) prior to removal so that you can refit it quickly and easily.
Good luck!
-Alex