what is the possibility someone has adjusted the accelerator pedal stop so that they think they get more pedal stroke (to get more out of the motor top end?)
if it is only happening under load when giving the accelerator pedal full power, you may be pushing the accelerator pedal position sensor (pedal pot) past what it is setup as "full stroke" and the "brain box" thinks there may be a problem with the pot.
Other things to consider are the vacuum lines going to the emissions controls. It's possible that in the vacuum line elbow right at the airway tube that mounts on the valve cover, there could be a "bb" sized ball that can wear a groove and get caught on it's seat, but that usually causes the engine to totally stop running. Some of these units had that elbow (It's black plastic, one side is a male 1/8th inch pipe thread, other side a hose barb, from the outside it looks just like the elbow that screws into the LPG [Impco] regulator)
Also, if the elbow on the regulator is screwed in more than finger tight, that elbow may be hitting on the diaphragm and not allowing the diaphragm to travel as far as it needs to, and the engine brain box thinks it can not adjust the mixture to compensate correctly.
I would also confirm that the vacuum lines from the throttle body did not get swapped around, so that the line that goes to the Distributor vacuum advance is the lower one (It should be the upper one) and I would also make sure there is no obstruction in the ports to those lines inside the throttle body (I use a welding tip cleaner to clean those ports when needed).