I ran into a smiliar problem. I put a 78 motor with single magnetic breaker ign. distributor into a 76 car body. Apparently the 76 car computer had dual magnetic breakers, and a computer which would actually warm up the car through one electric circuit of the computer for the first five minutes, (probably the retard) and then switches to the advance once the engine is warm. (Its been awhile, but I think their is a temp. sensor involved?) Anyway when I installed my motor, I had hooked up only the single retard power line to the distributor. Once the car warmed up, it would switch to the advance through the computer, and cut off all electric juice to the distributor. On a 76, I believe their is a green & brown wire to the distributor dual breakers. If your running a late single wire distributor, you may have the same problem as I experienced. If you have the stock distributor, it may be possibly that the advance breaker isn't working, or the wire has a break in it. To trick mine into running, I wired the retard & advance wires together, so no matter what the computer wanted to do, I always have juice at the distributor. Maybe this helps?

Speed.