Is the expansion tank cap on securely?
I know this sounds like a dumb question, but when I flushed my coolant for the first time, I had closed the cap to where is clicked on the detents on the expansion tank spout. I had high temp problems and coolant overflow, collapsed hose from the coolant pump, etc., for about a week until it dawned on me that the cap would rattle when the coolant overflowed...so I twisted the cap some more and presto! Problems disappeared!
Sometimes the simple solutions are the ones that work!