[old_master]

Most likely a worn distributor drive gear or bushing. It's caused by the distributor not being installed "plumb" in the bore. The inside diameter of the intake manifold bore is larger than the outside diameter of the distributor housing. This allows camshaft retard to be adjusted. Dorman makes a new design aluminum distributor, stay away from the plastic ones. After replacement, perform the crankshaft position sensor relearn, and then check and adjust camshaft retard, (both require a high end scan tool). If you just replace the distributor, the same thing will happen to the new distributor. When you cleaned the terminals inside the cap, you removed the silicone dielectric grease. When the cap is replaced, a dab of silicone dielectric grease is applied to each terminal. When the engine is running, the electrical current passing from the rotor to the cap crystalizes the grease, (it's supposed to be there).