Based on the stated experience of the builder, I will assume it's not a clearance issue, although I'd want to double check it. Seeing as how it's a two time occurance this is most likely not the problem.
I would guess an overheating situation that could be caused by blocked water passages in the front of the block or a failure to completely purge the cooling system of air on startup.
After I have a block tanked I use a piece of coat hanger or long screwdriver and some compressed air to ensure all the crud is out of the water passages. You'd be surprised at what comes out during this operation.
I've also heard of blocks coming from the tank, set down on their front side to drain/dry and sediment settling in the front passages. Then someone fails to clear them.