Have you checked your crankshaft pully for a worn or gouged out keyway? If someone removed the crank pully by removing the center bolt instead of the 6 8mm outer bolts, then replaced the crank pully and didn't torque it properly, you could have a loose pully, which would do what your describing. This is a common mistake when changing the timing belt, and results in a mangled keyway on the end of the crank, which in turn results in major timing fluctuations. Check it before you buy a distributor.