Steppers don't work like that. Additional loads won't make them get hotter.and I think over a period of time it's causing the motor to work too hard and causes the overheat.
Just wondering anyone elses thoughts on this please?
Most likely, you're current or voltage (or both) is set too high.
What kind of drives are you using, what voltage power supply, and what current?
And what is the current rating and inductance of your motors?