Q400 Captain inadvertently moved both condition levers below the 850 RPM detent then quickly returning them to the proper position. This causes both PEC's to drop off line and the checklist advises landing at the nearest suitable airport. Engine surging is experienced during approach but becomes stable prior to landing.

Narrative

On climb out; I inadvertently brought the condition levers back to just below the 850 RPM detent. I immediately placed them back to the 850 RPM detent. Even though it was only momentary; the propellers did surge; and both Propeller Electronic Control's (PEC's) dropped off line. We completed the checklist; which called for a landing as soon as practical; and returned to the departure airport. Dispatch and Maintenance Control were both notified; and I made two PA's for the passengers. The only unexpected event occurred on final. Both propeller RPM's began to slightly surge up and down; which was noticeable in the sound of the propellers; as well as a slight yawing back and forth; during the below the line portion of the landing checklist. This involves the flaps going to 35; the bleeds being switched to min; a power change; and an airspeed change; which caused the propellers to come off the overspeed governor. This lasted for 30-45 seconds; then both propeller RPM's remained stable for the remainder of the flight.