Verify your ignition setup and timing, then check to be sure your choke opens fully. You can disconnect the choke linkage at the side of the carb, and use wire or a piece of elastic to hold it open. Be sure to secure the choke rod out of the way so it does not jam the linkages.

The car may be a little harder to start without a linkage, and may require a little gas pedal to keep running for the first few seconds, but once running smoothly check and see if your power is back.

Another thing to consider is that you said they shop did "valve adjustments". There are no valve adjustments on a stock Pontiac. So that may also need to be looked at.