[Johnny Outdoors]

working in the business of fixing cars and trucks
we get over heating complaints like yours quite often
with the heat going up and the heater blowing cold air then everything is ok then it is not
usually, i say usually it is a bad head gasket
combustion is forced into the cooling system creating an air pocket
if you take it to a shop they should be able to test it pretty cheap
with a tester that sucks air out of the rad and checks for co and turns the dye blue from yellow
the test takes less than 1/2 hour usually
good luck