timing issues

Did you replace the cam gear with a steel one? As for it acting up, do you have it connected to full manifold vacuum or ported (vacuum only when the throttle is opened) As for the under load part, that could also be the gap and dwell being off. There's a narrow range that works with points when it comes to gap and dwell. One affects the other also. Best thing to do, is to replace the points with an electronic module. And get a steel gear on there, otherwise the gear it came with will be toast in no time.

Dwell is more important than the gap. If it's not right, your motor won't like it. Twenty years back when i had the 67 427 Stang I hated screwing with points. And not all were the same. Accell and Mallory had the best sets. Heavier the spring, the more RPM's you could get. But it shortened the life of the set. A good set would last a 6 months to a year. Put a Pertronix module or a Crane XR-1 and forget about points, forever.