G-day MarkS
There are a few ways your timing can drastically change . Either the roll pin retaining the dissy drive gear has sheared and the gear has seized in a different position or the dissy clamp is not holding the dissy tight enough
if you can get the timing back to normal by adjusting the distributor then obviously the proiblem remains with the dissy and not the timing chain/gears. How far does it advance and does it advance to far if you increase the revs slowly and have you checked the vacum Regards Graham