After this first happened, you were able to correct it by resetting the ignition timing. This would seem to indicate that the valve timing was and is correct, and that the problem is not related to cam, crank or their respective sprockets. As others have mentioned, a distributor that is not properly clamped in place could cause this, and in this case you would be able to see (but may perhaps not notice) that the distributor had moved. I am wondering if perhaps the aux sprocket is not properly registered to the aux shaft (sheared dowel / Woodruff key; don't know what is used here). There is a bit of power transmitted to the aux shaft, as it drives the oil pump, so the sprocket could easily be slipping on the shaft if it is not properly secured.