[boynton]

part of the problem may not be the water flow in through your heater core but rather the air flowing through it. If the heater core is stopped up on the outside and air is flowing around it rather than through it this would also cause your problem. This is also a common problem with home heating/ac units. Debris such as dirt/hair will clog up the fins over time causing air to not be pulled through the core but around it. I know they make a spray to clean the debris but you would still have to remove it and if you go that far you might as well replace it. One solution might be to try to get an air blower on the core and blow the dirt off of it.

Good luck!