Small Engines (Lawn Mowers, etc.): loosing power, impeller, water pump

Question
I have a briggs model 135232 on a water pump. After running for 20 minutes or so it looses power-rpms. What can cause this?

Answer
How old is the pump?  Often times on pumps the main impeller bushing will wear.  As the pump heats up during use the busing will start to bind causing the engine to work harder to turn the pump and thus the engine appears to loose power.

When was the pump last serviced?
Eric