Motor malfunction...

Our motors stop running after a minute of usage. Then they start back on about 30 sec later. Why is this happening?

Are they attached to anything or are you just holding them in the air with no load? Most likely they are attempting to move some load and they are overheating, either at the motor itself or at the microcontroller

our motors are attached to sprockets for threads, if it is overloading then how can we fix it (we are even using a power expander).

Could be a couple thingsā€¦

Is one of the motors working against the others?
Is there a lot of friction in the system that the motors are trying to overcome? Treads too tight? Shafts misaligned? Bearing flats deformed?
Is the robot too heavy? Is it occurring while pushing or carrying a MOGO?

If your robot is carrying a lot of weight, motors can cut out, especially turbos, so if they are turbo motors, thats probably your problem