If you got the same make of cap, then the rotor should be the variable. See if you could find a snap in rubber plug, and drill the hole accordingly. Then you could always spot check it if the ignition starts acting weird again.
But before that, are the distributor shaft bushings sloppy? That can make your timing bounce. You'd need to remove the distributor and really check it closely, unless you already have. Also, try checking your timing with the vac advance disconnected and the hose plugged, see if it still bounces. Does the timing jump as the rpm's are climbing, or only when it's hanging at 1500? Just curious if possibly your centrifugal weights are trying to kick in right at that point, and you have a sticky spot. Also, does the timing still jump at rpm's above 1500?

 

problem was a bad msd box....thanks for all your inputs....

 

So how did you find out?

 

Well, I just traced the problem backwards. It didn't have this problem before the install and all the connections were correct. I didn't perform the rotor to cap phasing as mentioned because I didn't have that problem before the install. I just removed it and problem fixed.

 

pm sent