I've had this problem for some time now and it is not the Distributor because three of them have done it. When checking maximum advance when it reach the max which has been 20-24 depending on the distributor, the timing starts to jump around at least 4 or
5 degrees, about 2 up and 2 down from the max. The engine has 8000 miles on it, the distributors were rebuilt and the latest one is brand new. The vacuum is disconnected and I have bypassed my J&S knock sensor. I'm not sure if it did it when the engine was new but I don't think it did. Any ideas on what to check? ThanksAlex