I have a 2001 Toyota Camry with 187,000 miles. My temp gauge moves into the red area when driving up hills or idling for long periods of time. If it gets hot enough the coolant backs into the overflow tank. Have replaced sensor that controls the fan, thermostat, water pump, had the radiator flushed and replaced coolant. Sometimes gauge will go back to normal very quickly but later rise again. Spark plugs are coolant free and no noticeable leaks and antifreeze level remains the same. What could be wrong? Cost?