I can't believe I know the answer to this. Some models of Chevrolet Suburban have a govenor shift on them, meaning that if you go over a certain speed, the speed shuts off and goes down real low before it kicks back in to where you can drive it at normal speeds. If you are driving it at a high rate of speed, this could be the answer OR there is a computer chip problem where this govenor (meaning it controls your car speed) is kicking in at the wrong speeds. On google I typed in "where is the governor on a Chevrolet Suburban" and it explained it more . I hope this is your answer. The reason I know this was because of the Tupac Shakir slaying. His friends were trying to chase the car who had the ones who shot Tupac,, but their suburban kept doing what your car is doing when they would speed up too fast. Chevy did this to their suburbans for just a few years but this sounds as though this is what's going on. Please check out the site I wrote above. Sorry I don't know how to cut and paste sites yet.
This could be because of a fuel vapor or emission system fault. You need get the code just to be sure that issue with your vehicle is correctly. You may contact a professional mechanic for more inquires concerning the issue.