Well based on the info you are giving, my first thought would be possibly the MAF. Maybe it needs to be cleaned. Could also be the fuel filter, though that I believe would cause the issue at any speed rather than the ones you describe. Could also be an EGR issue, it may need to be cleaned or is failing. There are several things that could be suspect though.
Are you getting a check engine light or anything like that.
Also, though not an apples to apples comparison, the '89 Grand Prix I had did this same thing and it turned out to be the crankshaft position sensor, but the symptoms you are describing I have not seen associated with a crankshaft position sensor of any of the newer GP's, particularly the supercharged ones.
