Why are newer DASM motors more sensitive to load fluctuations? Asynchronous motor?

An IE1 electric motor (three-phase asynchronous motor) Replaced by a newer model with the same performance but IE3 . So same control (FU), same housing, same ventilation/cooling, same speed… The only difference is that the new motor overheated after 10-30 minutes , while the old motor ran continuously without any problems (without getting noticeably hot)….