[700rpm]

You should have gotten more than 12k miles on a rebuild with good babbitt. If the babbitt failed in so short a time, something was not done correctly. If you know who rebuilt the engine, you should go back to him and question him about the rebuild.

If you can't do that, or get no satisfaction from the rebuilder, I would pull the engine and tear it down to find out what else, if anything, is wrong. I would be especially attentive to the condition of the babbitt in the mains.