If possible, recreate the conditions when you have this problem, then in a safe place see if increasing the idle offsets the issue.
When the pump was replaced was the first pumps pulley reused? Was there a keyway to keep the pulley engaged on the shaft of the pump? A flat that needs a set screw to be tightened? Or a flat spot on the pulley bore that is damaged and allowing the pulley to slip on the shaft? Do you have a diameter measurement on that pulley?