hi you could have an air lock in the system, or a blocked heater matrix (small type or radiator in the heater unit)
first get the car up to temperature then feel the radiator hoses and the heater hoses to see if any are cold, they should all be hot always turn on the heater to full and to the hot possition after changing a stat while trying to get the air out of the system.
if you find that one of the heater hoses is cold it may be the matrix which is blocked, i usually squeeze all the hoses with the expansion tank cap off to try and force the airlock to move but the car has to be up to temperature and the heater turned to hot to ensure the circuit is open with no restrictions
hope this has helped
simon, February 2009
