What power source do you have your distributor hooked up to?
The original points & condenser distributor was supplied by a resistance wire.
You don't want to use that wire for your HEI, it wants a full 12 volt source.
Could be a weak spark issue at higher rpm, weird that it showed as a problem later though.
But stranger things than that have been known to happen.

 

Hey r now that is a real hot rod bro.

 

Hooked up a tach and it runs strong and starts laying down around 4000 rpm-- heading up to kentucky to wipe out an al-kaida cell of deer that have been menacing the locals-- will do some more things when i get back--

 

We got them out here in the West also.