(Read it twice before you post rusty)
Seems like an intermitently variating clutch would cuase the engine to surge and lurch and not idle properly.
Also if the idle is set slightly fast the clutch will grab and release causing lurching.
Remove the belt, grease the _clutch_ and spray some teflon or white grease where the balls are, work the pulley in and out by hand till it moves smoothly, wipe off any excess lube, let it dry, put the belt back on and try it again.
Rusty