This only occurs in a certain situation. When the outside temperature is hot 90 - 95 degrees or higher, A/C running and I am in stop/go traffic or parked in idle. The temp gauge rises above the half-way mark and the car becomes sluggish on acceleration. The check engine light does NOT come on. I have done the following recently with the same result: New water pump, thermostat, chg coolant. What else should I check?