The symptoms match a bad thermostat also.
They also tell me coolant flow is the problem, and it could very well be the water pump but I sure would swap that thermostat out for a tested working one first.
Standard water pumps have 7 impeller vanes and AC pumps have 8.
I opted for 8 since my car generally runs at lower RPM's I figured the extra vane would help the flow at lower speeds.
I also have seen those pumps where the impeller came loose from the pump shaft.
(They kind of still work, but act like you say yours is acting)
