Been following all this along and letting everyone else do the process of elimination like--faulty coil, carbon tracking in the distributor cap, bad cap to coil connection, distributor bearings, roll pin, etc,etc.
So my fix is to replace the electronic module in the distributor as well as examining the 8 point reluctor for cracks etc.
In my experience, when ever anything like timing changing, working and not working and unpredictable things happening that don't make sense, it always involves some stupid electronic device. Or in maybe this case, replace the whole distributor with another one, but yes as Mex says--get the advance curves correct for that engine.
