Seems like a intermitently variation clutch would cuase the engine to surge and lurch and not idle properly.
Also if the idle is set slightly too fast the clutch will grab and release causing lurching.
Remove the belt, grease it and spray some teflon or white grease where the balls are, work the pulley in and out by hand toll it moved smoothly, wipe off any excess lube, let it dry, put the belt back on and try it again.
Rusty