I've had the heater core go out 3 times in the past 12 months on my 96 Tahoe. For those of yall that have experienced this you know that it is a pretty annoying problem because of the mess that is created by the antifreeze leaking into the interior of the vehicle on the passenger side.

My question is why does this happen? Why do they keep going out? This last time my mechanic said he flushed the system very well so hopefully it won't happen again, but I guess I'm skeptical. I do know that I had converted it from the newer orange antifreeze, back to the old green at one point in time thinking I'd save money. I think I did this at a point when I had to replace the AC, it's been a while though. Is it possible that this is the reason why I've had such problems with my heater cores going out in the past year? Any information on this topic would be greatly appreciated!
Thanks a lot,
Conrad