had a similar problem once on my old '68 dart 340.

it turned out to be...

the distributor cap had a hairline crack. you could only see the crack if you held it up to the light.

common issue if you recently changed the headers (jacked up the engine) or broke a motor mount etc because the smallblock dizzy will hit the firewall

yours could be a different problem. but it's like a $5 / 5minute fix
worth a try