Bursts of acceleration

I have a 1999 GMC Jimmy and every now and then I will be cruising at a constant speed and my car will give a short burst of acceleration....kind of like something is clogged and it's pushed it out. I just had the spark plugs replaced, transmission, and radiatior flushed, fuel filter replaced..etc. Does anyone have any suggestions on what to do? Any help would be highly appreciated b/c of the fact that i just spent 500.00 on it. Thanx.

Since I have posted this, I have gotten a number of suggestions from different mechanics such as: A bad speed sensor, a selonoid problem, etc...anyone ever have this happen to them?