Engine Cooling problem
1999 Chevy Venture 6 cyl Front Wheel Drive Automatic
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we have replaced the whole cooling system. radiator, waterpump, hoses, radiator cap, and thermostate. the mechanic has rereplaced the thermastat and tried a few other things. the vehicle temp. gauge says its overheating and the computer shuts it down to a couple cylinders to maintain movement. when it is running alright the gauage bounces from normal to hot. we never have to add coolant but yet this is still happening. the mechanic is stumped. the chevy dealer wants to replace rear gaskets for 1400.00 dollars but they dont know ethier. i need something cheaper than a guess.is there possible a sensor that is bad or something else?

Could be the coolant temperature sensor investigate/test