I have the same issue!! It almost feels as if it is skipping. It is not a misfire for me but under heavier than normal acceleration, usually in 3rd, 4th, or 5th gear it does this. I have not been able to find a pattern either. I rarely run my A/C or heater so its not that.
Changed the coil pack
cleaned the throttle body
checked air filter (Clean)
replace thermostat and coolant
replaced serpentine belt (this is is the last thing I have done and thought that maybe the belt was slipping. I replaced it because it was squealing when it was cold)
The only thing I can think of is my cat is going back because I have a recurring P0420 code coming up. I think this because it is my understanding the Air/Fuel ratios are influenced by the O2 sensors and my downstream sensor closely follows my upstream sensor. I am hoping that when I replace the cat it will go away, that's just a hope though.
newb - 2005 Cobalt, Base Model, 5 speed. 2.2 Ecotech