Engine cutting out under load

[Don-Ohio]

As I was driving a month ago, the engine started "missing" under load as the engine wound out. It really felt like when the spark wasn't advancing) I figured timing/spark advance, but it looked fine. So I looked at carb, thinking maybe it was starving. put on spare carb; but same problem. Head/ manifold gaskets are fine, valves are a little out( a few thousanths bigger gap than needed) so not enough to cause this. I even took muffler off in case an critter plugged it up. The engine was rebuilt 2000 miles ago and the only thing left I can think of is some kind of spark problem (new plugs put in )... Any ideas?? The engine runs pretty smooth at idle and at an easy pace, but as soon as it winds out or going up a hill, it just is cuts out.