Temp Gauge Erratic '73 Dart (225) /6

Well, I have put 3350 miles on this little Dart since purchase in November without an ounce of trouble so I guess it is just time for a little head scratching and troubleshooting! The past couple of days I have noticed the temp gauge leaning toward the 1/2 way mark running down the road at 55 MPH or so. It has stayed down around the 1/4 mark since I began driving her on a daily basis and now the needle will run up to that 1/2 point and then drop back off? Stays down where it belongs at idle? Turning the heat on with the fan blowing seems to help. Coolant level is same as the day I bought her and I have not found any leaking around the water pump either. The only thing I have found that is really odd is that "U" shaped shelf facing forward on the water pump housing below the pulley/shaft has a sticky oil or grease puddle in it? I have a new thermostat so I may go ahead tomorrow and change it and see what happens. This afternoon the needle moved petty quick upwards then dropped back off just as quick without me doing anything wit the speed or heater fan. Sending unit? Thanks in advance! Mike B.