hmm so no one can think of any reason for this happening twice in one month other than just bad luck?

Well...

I did a lot of searching and found a used replacement at a junk yard and installed it my self.


You bought a used heater core to replace a busted one? We are talking about 15yo vehicles. There is only so much life in some parts. Going cheap and replacing something like a heater core with a used part usually ends up costing more than just finding a new one. Now you are out your time, the cost of the used one, plus the time and cost to replace it again.

That, is why it happened again so soon. There is a remote chance that you have a head gasket leak that is pressurizing the system, but the radiator or cap would proably let go before the HC. I would place much more faith in the above explanation.

Good Luck,
Michael