I have a 2006 land rover LR3 with 230000 miles that is overheating. I replaced the water pump,thermostat and replaced all old hoses and the radiator. I replaced the cooling fan clutch . I drained and flushed the cooling system and filled with proper coolant. Now after about 20 minutes at idle (650 rpm) the RPM jumps to 1200 and the temp gauge goes straight up. I turn it off and back on and the temp gauge drops back to mid-range immediately and then for about 5 minutes then RPM goes back up to 1200 and the temp gauge pegs hot again. Any ideas?