Possible that the intake gasket slipped out of place causing a major vacuum leak. BTW a propane torch ( small hand held) can be used to locate and confirm vacuum leaks by adapting a hose over the nozzle, and holding it over suspected areas, including the throttle body intake, this will richen the engine causing it to smooth out when you are near the leak, tank is not lit of course. Something was done wrong in the repair process, focus on those steps. Cam could be out of time also. I assume no bent valves from the pushrods being installed wrong, correct?