its possible that your mixture screw sometimes known as the low idle screw is out of adjustment . if this is out of adjustment then the high speed screw becomes very touchy for getting the right mixture and the engine varies with different throttle settings. Check to see how many turns the mixture screw is out. if it is far from factory setting then reset the screw. to factory settings. I wasted about an hour dead sticking a friends plane until I decided to check on the low speed adjustment screw.