[rickj]

One additional thought: since I'm somewhere between cheap and frugal I might in your situation take the following approach to hedging my bets. If the corrosion is external as it appears and is limited in area I might have a good radiator shop close off the bad tubes as long as most of the cooling area remains, this is a judgement call. I would not replace the radiator until I was sure I had found the cause of the corrosion. I would probably do this even if I had to take it slow up some hills on a hot day until I knew for sure the problem was solved. I hate these problems with ambiguous causes.

Jon, I like that approach, and my engine runs cool anyway so I doubt I would overheat on a hill. And if the engine runs in a low temp range I might just leave those tubes blocked off(expense on this issue is already out of hand). And as you state this provides me with the time to determine for sure what is going on; although I suspect it is electrolysis caused by 'stray' current (as a former boater I am all too aware of how much damage electrolysis can cause). Again, thanks for your input.