I sometimes leave the power at 1000RPM & apply brakes to slow down, then release the brakes but this is frowned upon by some instructors.
A bad habit - it will cause brake wear and might even overheat the brakes. If you need to slow down, use idle power first. Do
not use power against brakes.
If the engine sounds lumpy at 6-700 rpm that's normal. If it stops running, it should be adjusted.