check for timing chain slop, anything more than a 1/4 inch is too much. try disconnecting the vacumn advance then see if it jumps around. it could be that its starting to pull the distributor at idle.
i assume that is jumps around at all rpms? or just at idle? Also check the reluctor on the distributor. I had a jumping around problem once turned out the reluctor on the distributor looked like a bunch of mice had been chewing it. Changed the reluctor and the jumping stopped.
how many miles on the motor?
Have you tried connecting the light to another spark plug wire and see if it still jumps? Possibly something wrong with #1 plug wire .
