I am driving a dc motor with MosFet and when i read the volt out clamp to the motor is up and down so me robot a fast and slow ...

Example : out v is shut be 2.0 and not 2.1 but after liter of time change to 2.1 so my robot run i liter fast for a second and then go back so is not stable

my source is 7.2v


my MOSFET is Rectifier IRLU3103
I use 10K resistor, with a 12K resistor gate to ground. Higher gate to ground

resistance reduces resolution of the byte Duty Cycle variable.