Start engine and it revs higher than normal (1800-2000). It takes longer than normal to heat up to norm op, then almost over heats. I rev the engine and the heater starts blowing warm air finally and temp gage comes back down. If i stop or let revs drop, process starts over. Even when gage reads norm temp, heater blows cold. I changed thermostat, added coolant (low), added oil (low). No joy. What should I do next?