Hoping someone can give me a little insight. AC works ok but I have no heat. Used a temperature gun to check the lines and the two going to the heater core are hot (about 120), but the one coming out is at a much lower temp (76 degrees).

I had previously several coolant issues that have all since been repaired last winter, including new aux water pump and heater valve. The heat didn't work then either and I can't really recall the last time it did work.(the winter before that I was stationed in SC and didn't have the car with me)
I've been reading on - JaguarClimateControl.com but not sure what direction to look next. I'm no expert but is there a way to check the heater core for blockage?

I have an '03 STR.

Edit: My uncle tested the lines again to and from the heater core and says the return line is over 100 now. The air isn't blowing hot, just a mild warm temp right now. But it's usually just blowing cold.