Mine had the same issue after rebuilding the motor and I couldn't tell because my rpms were almost dead on but then I thought about it, my buddy and I worked together and never will he touch my car again because he doesn't really tighten anything down right. My throttle cable was never tighten back up and it ended up slipping a little and after I adjusted it the problem went away. It was barely noticeable because I could push the throttle cable and not feel a difference but the cable had just enough slack to slip a little bit and that's all it took. Make sure it's tight and adjusted correctly. I could very easily be wrong, there's more options to look at other than that but it never hurts to check and be sure. Hopefully this helps you out