Just as a clarification, if your WP was going bad, the temp variations wouldn't last very long - AFAIK that warning sign only lasts a couple of days at most (then it will go into the red and stay there when the pump fails completely). Check your heater to see if it's blowing hot (tough to tell in hot weather, unfortunately). Also, if you do replace the WP, don't forget to throw in a new thermostat as well (it's nearby and you might as well do it since you'll have some coolant drained).