I did the test as described above and with advance on and with it off the dwell starts at 30 and with the throttle increased fast or slow the dwell goes to 31Can you duplicate the condition by just reving the engine to the problem rpm area? If so,put your dwell meter onit and watch it's reaction.If it gets irattic,then disconnect the vacuum advance and repeat the test. If it stablizes,then look at the ground wire that runs from the advance plate to one of the screws that hold the vacuum advance.Also,if your points set is one of those "uniset" units,DUMP IT!It sounds like you may be having a situation where that dist.is loosing it's ground.
You did not do it correctly , my fault. You have to get rpm where it bogs and hold rpm there.. or you can get a long hose routed to driver seat and drive it to the rpm bog and observe guage which might be easier .Exhaust off does not change the problem. When I put vaccum gauge on it (with exhaust on) it idles with 16 to 18 inches and if I open it up it monentarilay dips to five then increases. When the plates are closed with engine rpm going down it is at 24 inches
What I am implying is that the exhaust could only be restricted enough to get to 3000 rpm ,after that it is not open completely to full rpm .Do the driving test ,that will tell you .As you say it starts at 3000 consistent rpm. A short burst of acceleration will not be a true vacuum test.when it bogs the vacuum will decrease lower and stay there while it is bogging