The cast impellars are supposed to work better, but the aftermarket stamped impellars do fine, too. Sounds like it may be a timing problem. What is your timing set at?

Fresh rebuild, it is possible that the tolerances are still a little tight as well.

Good luck! I JUST cured my overheating problem at idle. Turns out the impellar would spin freely on the pump shaft. I would get some coolant flow until the engine started getting warm, then it would bubble and spew. The water mixture was boiling in the heads.

Fixed the pump and now there are no ill effects. Runs at 180 in 98 degree Tx heat and high humidity.