Not sure what is happening here but we just installed a new motor in my '79 and it runs fine up till you get around 3,600 RPM's then it gets all choppy??? Any ideas? Not hearing any vacuum leaks and we have checked all the obvious things and running short on ideas here?

Any help would be greatly appreciated.

Have you checked or readjusted your rockers scence break in? Not to get on the bad side of this, but i would pull the valve covers and see if your rockers are all moving the same. This would be a good time to adjust them and make sure you don't have a cam lobe going flat. Secnce we don't know what has been done to the engine or how you broke it in, its hard to tell.
Timing? Carb adjustments? Jetting? Fuel pump? Have you done anything to the disrib? Are you a tooth off when you restabed the distrib.

Are you having a lean pop? Is it poping throught the exhaust or back through the intake?