What carb are you running? When I had the a12/Hitachi in my car it had the same problem all of a sudden. Took out the air bleeds(gotta take off the carb top to do this if I remember correctly), after careful examination, a tiny piece of gasket was lodged in the small hole. When it was removed, the engine purred once again!
Also, check your distributer advance isn't loose/sticky or something.
When in doubt, I always go back to the basics; check for steady vaccum and do a compression check.
Are you running big valve springs? Double ones? May help to pull the valve cover and check to make sure the lobes aren't chasing the base circle.....going flat.
Hope this helps a little.
-Donnie