Bob,
somewhere you have a vacum leak. Intake manifold to head, carb., there is a hole on the side of the manifold for a vacum source for power brakes/pcv is it plugged or open needs to be plugged.Since you had the same problem in 1993 with the engine I would think you need to step back and look at the simple things first.
The intake/exhaust manifold: this is a two piece unit that bolts together under the carb with 4 bolts. I don't know if when you rebuilt the engine you seperated them, but if you did they may not be straight, alowing for a vacum leak. You can check this by spraying some carb cleaner/ ether around the intake to cylinder head surface and if the engine speeds up you found your vacum leak. If that is the case, remove the manifold and replace the gaskets, however, you need to check for a straight surface on the manifolds, loosen the 4 bolts between the intake/exhaust manifold, a gasket should come with the gasket set, and change the gasket, reassemble but don't tighten these 4 bolts untill after installing on the engine, this will allow the manifolds to fit tightly to the head, then tighten the 4 bolts.
Timing: like Bartamos said, sometimes the ring on the balancer will slip, especially if the balancer was installed by the hammer method( beating on it) instead of using a bolt in the center of the crank to pull it on. Try turning the distributor and see if that makes a difference. Check the vacum advance, pull the line off and suck on it if the dist advances your ok if not you need a new advance.
These are simple well built engines it will turn out to be something simple, we all have had these same gremlins over the years and will help you through this.
