A couple of things come to mind. When they did the intake manifold, they might not have gotten a good seal and there might be a vacuum leak. That will cause a surge. Or, when they put the distributor back in they may have set the timing a little retarded from where it was before. Again, that will cause a surge. As for setting the mixture, when the motor is warm and ideling, turn out the mixture screws one at a time until you have the highest RPM possible, then turn in, a quarter of a turn at a time, until you hear the RPM's drop, then turn the screw back out just until you get the lost RPM back, no further. Now do the same thing to the screw on the other side. I usually go back and forth a few time to get it just right. There are lots of reasons a motor might have a surge, but I am trying to think of ones that might have come from changing an intake gasket. Hope this helps.