[twobjshelbys]

I had the same problem - very erratic temp swings over very short intervals.

At first I thought air bubble too. I got one of the Kirkham tanks which successfully raised the fill point to the high point of the system. It still happened.

Eventually we'd tried everything, including thermostats and various caps, and the problem ended up being the water pump itself - the pulley and everything were turning but the impeller itself was broken and not spinning - the impeller shaft connection had broken loose and was just spinning. The blades could line up to a dead spot where no water would flow then it would take a little shift which would then flow a little while.

This was after everyone assured me that if the pump would spin that all was well.