340 problems

I don`t know if you`re running a vacuum advance or not but you might try it without just to see how it reacts. Check your timing with a different timing light than what you`ve been using (may be the light). Also the initial timing may be too advanced which would cause high idle and breaking up at higher rpm when all the timings in. If you have a MP distributor check the advance in the distributor at 3000 rpm. I have one that picked up 3 more degrees from 2500-3000.