Heater core?

Same thing happened to me, check out my build thread, lots of pics in there.
The old core was pressure tested before as well, but it still developed a leak a few days after i had the car running and driving.
I changed the heater core, and now the car is problem free. It's not a bad job to do this with the dash still in the car.
BTW, i used a spectra brand one, and i had to modify the heater box to install it, not a big deal, but the inlet and outlet pipes were off a little bit........
Just a little heads up, i don't know about other brands.