Ive seen motors at very low load (e.g. 10%) get hot because their efficiency drops like a stone and their power factor goes to crap. Also, incorrectly wired motors run hot, and motors with extra resistance because of really bad seals, alignment issues, or internal issues (including in the pump) can run hot. It would be good to start with the pumps flow, pressure, voltage, current, and power factor data to make sure there isnt an obvious pump or motor issue. If youre off the pump curve or in the cavitation range then you know you need to change either the pump or piping system. But sure, you could go out there and start "trying stuff" until you get it working suitably. But wouldnt it be better if you looked at the pump curve that you're dealing with so that you know ahead of time where the pump will operate comfortably and where it will not?