check to see if all your cylenders are firing correctly it sounds like it is missing on one, start by removing and replacing your wires one bye one the one that you pull that doesnt effect the idle is what your looking for. next check your compresion in that cylender you shouldnt have anything less than 90 psi in it and even 90 is low but thats what it needs to fire correctly. if you have compresson then check wires and plugs. good luck