Strange SBC problem.

Here is an easy check for you. Check the ground cable from the battery if it is loose or corroded it will not supply enough voltage to the dist. and will create the same symptoms you are experiencing. Just my 2 cents/ Larry

 

Sounds like the distibutor is off one tooth. make sure its lined up with #1 .And make sure the balancer didnt move to make the marks off a little.

 

If it hasn't been mentioned I have seen #5 and #7 wires mixed up more than once......