1996 Ford Thunderbird temperature gauge fluctuates

1996 Ford Thunderbird mileage: 56,000. The problem I had was the temperature gauge would fluctuate going very fast to hot, stay for a minute or so then back to cold very fast. My mechanic changed thermostat. It worked for about a day, I took it back to Mechanic three times with the same problem. Each time he changed the thermostat. This happened about three times. Finally, he installed thermostat for a turbo charged 3.8 engine. The car now takes a long time to heat up. Do you have any knowledge of what is going on with this car? My mechanic is at a loss and he does not know what else to do. He even consulted with the Ford people. If you have any explanation on this, it would be very greatly appreciated.