Actually, that tells me the heater core is good and NOT clogged. The heater core is nothing more than a small radiator. When you turn the heat on, hot coolant from the engine runs through it. WHen you turn the fan on, it blows through the heater core and blows hot air into the vehicle. If the temp drops when the heater is turned on, that tells me coolant is flowing through the heater core, cooling off, and then helping cool the engine.
At this point, here is what I suggest. Since the temp gauge shows that the engine gets hot then cools, that tells me a couple possible things. First, it sounds like an air pocket in the cooling system that is slowing coolant flow. Have them try to rebleed the system. Next, if that doesn't help, have them replace the thermostat. Keep in mind, the old radiator would not have had the same flow as the new one. Consequently, the temp would have run hotter and the heat would have been hotter. Now, how can that be fixed? The thermostat may not be working properly. So, after trying to bleed the system, if it doesn't help, have the thermostat replaced to see if that levels out the heating up issue. While the thermostat is out, have the mechanic flush the engine block and both forward and reverse flush the heater core. NOTE: It is important that he reverse flushes it too. After all of that, if the engine still overheats and you get poor heat in the vehicle, my next guess is you have a bad headgasket. ONE LAST THING. Before they start into expensive repairs such as a headgasket, have them replace the temp sending unit to make sure it isn't sending a false signal to the temp gauge in the vehicle. The engine may not be overheating at all, but rather you are getting a bad reading in the vehicle.
Let me know how things work out for you and if you have other questions.
Monday, January 10th, 2011 AT 1:15 PM