[ursus]

I had a Model A that jumped time every time I hit a bump or hit the brakes. I would then have to pull over and reset the timing again. I went over the distributor several times, switched to another distributor known to be good, but the problem persisted.

I later found that the spring inside the camshaft thrust plunger had broken due to rust. This allowed the camshaft to move back and forth resulting in variable ignition timing. When the timing was reset, the act of pulling back to take up slack on the distributor shaft moved the camshaft (and its drive gear) back to the correct position. After resetting the timing, the engine ran fine until I hit a bump, sneezed, scratched my bum, or had impure thoughts. Check that out as a possible cause.