[77newell]

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.