Did you verify that the motor and wet end are matched up, it sounds like the motor and impeller are mismatched - a common problem when not replacing the motor with an identical unit - HP is not the only spec in the equation. Seeing that it's draw is only 4A with no load, it looks like it's trying to push too much volume once under load, straining the motor - this could be because of a mismatch between the inpeller and motor.
You did not state whether the original motor was replaced with an identical unit, only that hte replacement was replaced with an identical unit - this is what's leading me to believe you're dealing with a mismatched motor.