Diodes in my '79 keep failing. This is the third time in a few hundred miles. The electric shop keeps fixing it, at about $100 a pop. They say that nothing specific is causing it. My mechanic says it's because the car sits during the winter. I start it periodically, so I have trouble accepting that. I've read about converting to a later alternator, but I'm electrically pretty inept and don't understand the explanation. Is there anything I can do to stop peeling off hundreds chasing this problem?

Thanks, Rob