Dodge: 99 Dakota high temp, no heat, magnum engine, clutch fan

Question
5.2L magnum engine. This has been an intermittent problem for 2 months now.On occassions, temp. guage surges to high end and no heat from heater. Example: 10 mile trip, everything fine. Park for 3 minutes idling, temp guage climbs, no heat. Turn vehicle off. Restart temp guage plunges down and viola, heat. Another trip...115 miles, no problems.
Just replaced thermostat with negative results. Low milage (28,000). Old fluid somewhat tainted brown but tests out at -34 protection level.
Truck has tow pkg (tranny cooler). Am I looking at a temp. sensor switch problem, or maybe a backflush from a clogged rad. or heater core or a broken H2O impeller?
Serpentine looks o.k., no noise at pump, clutch fan seems o.k.
Thanks, Charlie  

Answer
I would check to make sure the radiator itself is not clogged up with debris.Make sure the inside of the radiator isn't corroded.Make sure the fan blades are not broken.And either have the radiator cap pressure tested or buy a new one.Did you make sure the thermostat was in right?As far as no heat touch the hoses that run to the heater core and see if they get hot.If they do your heater core may be clogged.