Whirlpool Duet Washing Machine Stops during Spin Cycle

I just started having a problem with my Whirlpool Duet washing machine. Only during the spin cycle, the machine stops with 3 minutes left and indicates an F-11 error message. During the spin cycle, the machine violently rattles and starts moving all over the place.

I had a Whirlpool certified repair technition look at my machine. he cleaned the pump as I am on well water, and he could not identify the source of the F-11 error message or why the machine stops during the spin cycle.

Sine he cleaned the pump, the machine has stopped 50% of the time during the spin cycle only over the last 40 washes. I would appreciate all comments and suggestions.