2004 Buick Rainier Studders?

I have a 2004 Buick Rainier CXL, 6 cyl. w/35,000 miles... It has always studdered slightly when I accelerate from a dead stop. The service department changes the code in the computer, but it does not work. Anybody know if this is a problem in Buicks/GMs? Is it bad injectors or timing?