97' deville concour coolant system-Mystery?????
 

97' deville concour: temp. guage goes red.............the dealership has replaced the following items but for some unknown reason the temp. guage goes in the red if you get the car out on the hwy........if you drive the car just in town the guage stays half-way and holds but once as stated you get it out on the hwy. the temp. needle goes into the red...........then, if you wait shortly (1 minute or so) the guage drops back down to the half-way point.......we have taken this car into the dealership and have spent i don't know how much money but i can't figure it out.......here is what has been replaced gm brand new

REPLACED COOLANT OVERFLOW TANK, TEMP. ACTUATOR REPLACED, NEW RADIATOR, REPLACED COOLANT PIPES AT REAR OF ENGINE CROSSOVER GASKETS, NEW WATER PUMP AND HOUSING, NEW THERMOSTAT

I just don't get it...........the fans kick on also because i've checked.........we have an obdII code reader and the only thing it very, very, very, seldom pulls up is that some cylinders have shut down due to the engine becoming to hot............if anybody could please help i'd really appreciate it........car only has 97,000 miles and has been very, very, taken care of....