could be a t-stat that opens a little to soon, a plugged heater(or partially plugged) core could be a malfuntioning water valve if equipped, possibly even the fins came off on the water pump, but with my expierence with subys i would say you most likely have a issue with your t-stat, is your vehicle equipped with a temp guage and is it reading lower than normal? check your heater hoses for temp differences.
And if your in the scarborough area you can bring it by my shop it will take me all of 10 minutes to check this out for you and give you a proper answer.