[Alan Roehrich]

A converter will not cause a stumble. At least I haven't seen one do it in about 35 years or so. It will make one lazy, but not a real stumble.

As Danny said, it sounds a lot like a carburetor problem, both the Q-Jet and the Holley carburetors do it, but for different reasons.

It can also be one of any number of electrical problems, a connection, a loose ground, I've even seen a battery do it because a plate came loose. I've seen an oil filled coil do it, too.