Two heater cores in one year. Whats up with that?

Hello, I have a 91 stepside that gave me a good 10 years of service, but now I'm getting ready to put heater core #4 in. The last three (including the original) have all failed within the last 12 months. The first replacement was done at a dealership when they did the manifold gaskets. I did the second replacement and flushed and put in new fluid. I have read of others who have had the same problem. But I have never heard how they resolved the problem short of buying a new truck. I read an old post about bad grounds and current running in the coolant. Can anyone offer me a solution to this problem. I think I can get a new core for free, haven't check yet though. But, I want to fix it this time, and not just replace the core and wait another 6 months to replace it again. Thanks