Well,there ya go!Go get them checked out.....Pontiac Service Managers will know exactly what you're talking about(..at least they ought to).
Also,if the steering wheel vibrates/shutters in your hands while pulling away from a curb or backing into a parking spot or any other steering maneuver while the car is BAREly moving[...this happened in my wifes '01 Bonneville SE],........there is an upgrade to the power steering lines/hoses(they are thicker,stronger)that needs to be done.There was a Pontiac Bulletin on it and Pontiac will fix it for free[BTW,the Tech Service Bulletin for the problem is 99-02-32-007A,replace steering pressure and return lines(just looked up my invoice on the deal)]

Today I was getting a slight vibration through the steering wheel at highway speeds, and a shuttering in the wheel under heaving braking. The ride was also rough, paticularly as the rear went over bumps.

However, on the way home the problem was much less and even went away for a period of time.

The only difference was the outside temperature and the car had been in the sun all day. The had been sitting for a few days before hand in a parking deck.

Any ideas on what may be wrong, besides the recall?

Maybe need my tire pressure is low (from cooler temps), or tires need to be balanced?

Also the steering does feel sort of loose, so maybe it is the steering shaft?