Possibly one of the distributor caps or rotors are bad, this vintage was known for that. If one of the rotors is bad, that's exactly how it would act too (like running on 4 cylinders) because that's exactly what it would do. Or possibly a coil, there are 2 of them, so you would have the same result as a bad cap or rotor.
Is it temp or weather related at all? Only happen with the engine hot or cold?