ive had this issue a few times on different cars/trucks and flushing out just the heater core with a garden hose did the trick each time....i hook up a line from garden hose to inlet, then outlet to ground/bucket, turn hose on, wait until the water running out is clear, swap inlet for outlet and "backflush" it, keep going back and forth until its always nice and clean, hook lines back up, bleed cooling system, enjoy.