This happened to me once. Turned out I had an air pocket inside the engine so it was not completely full of coolant. When the engine got hot the thermostat would open and suck the coolant into the block. This made my coolant level look low and since there was not enough coolant anyway, it would get hot. Try taking off the radiator cap, let the engine run and as it heats up keep adding coolant. Once you have been at normal operating temp for a while and the rad is full you should be good. Hope this helps.
BTW: If this does not help, pull the thermostat and check it. I have gotten bad ones before and a buddy of mine even installed one upside down.
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"Dog-gone it Roy-Jean, how many times do I have to explain it to ya, when I tell ya to put a rock under the wheel, I MEAN A ROCK!!" ~ C.W. McCall