Temperature gauge on my 88 Chevy pickup (305 engine) goes into the red after about 15 minutes of driving first thing in the morning. After about 30 to 45 seconds the gauge goes back to normal and doesn’t cause any more problems unless I have to stop. After this happens about a half gallon of coolant has been lost. Thermostat has been changed. Problem comes and goes, sometimes doesn’t happen for a week or more, then happens every day.