2008 Malibu problems...can somebody please tell me what going on??

My wife has been having some problems with her 2008 Malibu LS for sometime now. If we are crusing down the road and start to accelerate you will feel a thud that will shake the car almost like it coming from the transmision or something and will not accelerate much at all when this happens. It will do this at speeds from 20-30mph up to 60mph. It doesn't do it all the time which is weird but to me it's seems to me like the car is trying to down shift into a lower gear and something is not letting it. I've also wondered if it could be the spark plugs, spark plug wires, or possibly the coil pack going bad.

Hope this makes sense since it's kinda hard to explain, has anybody had this problem before? Her car currently has 52K miles on it but it's supposed to have a 100K mile powertrain warranty through Chevrolet. Should they cover something like this if we try to take it in under warranty? I'm just affraid they will keep the car for a couple days and say they can't find anything wrong with it then try to give me a expensive bill like most stealerships do

If somebody could help me out it would be MUCH appreciated. Thanks for reading!

Thanks,
DwDrummer