please help idle out of control 900 to 1800 rpm on 1996 Toyota Corolla

my dear old 96 corolla 1.6 4A-FE, has a idle problem, it ranges from 900rpm to 1800rpm. and driving it is less than smooth between throttle on and off. almost as though it has a air leak. but none have been found, a friend has check the idle control valve and it seems to be ok. any other idea's

Kind regards Stu.

Asked by for the 1996 Toyota Corolla