If you are replacing heater cores that often make sure you go to the Kragen, Pep Boys or whatever the big chain auto store is and get the lifetime warranty. This way you can save money while you investigate the problem.
I'd make sure the pressures in the cooling system are ok (squeeze the cooling hose as it goes into the radiator at operating temp), but I think this would not likely cause metal to fatigue repeatedly, maybe one core can have a weak spot but not 2 or 3. My best guess is that the heater core is not installed properly. Make sure that when it's in it can not vibrate, shake, have a loose part hit against it, or be tweaked in any way. You can get that sticky 3/8" thick and 1/2" wide roles of rubber tape to help support it. Make sure nothing is rubbing on it as it passes through the firewall.
If it is a huge problem you could just bypass it. If you need defrost you can just cut a potato in half in rub in on the inside windows, that will prevent it from fogging up for a few hours.
