a system with air pockets can overheat. to burp it:
on a cold motor, remove the rad cap. jack up the front end so that the rad cap hole is the highest point in the system (jacking or creative parking eases the air's path out). run the motor like this til up to operating temps. be sure the heater is on while doing so.
you should see some bubbles coming out periodically, as well as seeing the t-stat open at the specified temp (water will flow from the pass side to drivers side if you peer down into the rad cap hole). be careful; dont get burned or anything.
once no more air is comin out, put the cap on and take it for a spin.
if that does not cure it, please be more specific - when does it overheat? are you using a real gauge? other symptoms? etc. good luck.
