How old are the plugs. It was worse in the old leaded fuel days, the plugs could look fine but would still miss fire under a heavy load. If you would take it out and hammer on it a few times in a row, it would clean up some and run better for s while till the plugs lead fouled again. It isn't as bad as it used to be but it still happens.
My 427FE Is Going My 65 Fastback!