I don't know if you have had your raditor checked, sounds like maybe the inner cooling tubes maybe clogged some. The other problem that i have found with this is sometimes the tube in the reseviour tank maybe cracked or split around the top where it goes into the tank. It will cause air being sucked into the readitor instead of water. IF you have flow, the water pump being good, the raditor being clean, the thermostat good ( this is something that i boil and check to make sure it opens at temp) you can get a thermo at wal-mart to put in the water that you are boiling to check for right temp, the head gaskets good, the fan working good - with a good temp sensor,(i had this problem just a couple of months ago) had to change it out. The problem should go away.
(unless your motor has a lot of crud in the water passage tubes and heads.) seen that too.
