I would be interested to know if there was a "Check Engine Light" as well. Is this an automatic or manual shift?
I am not 100% on this, but the other things sound like they could cause this, but most of the time bad plugs will show signs that they need to be changed by a rough idle, stumble in the idle every now and then, stumble at "high speeds", so on and so on.
Honestly, if it is an automatic, I think it is your Torque Convertor Clutch or possible a bad/worn band in the transmission.
Without getting into a ton of specifics, you may ask yourself, "But how and why?". Poor maintaince practices by the previous owners. Our autos require some rather aggresive maintaince practices when compared to most others. Just remember in the future, that you should have your tranny fluid changed at least every 12,000 miles. Most of us here have found that this is the easiest way to fend off these demons. Don't worry about the filter, it is not serviceable. It is located deep within the transmission case.
Since you are going to be calling the shop/dealer this morning, I will save you the worries of testing things yourself.
If it is the tranny, don't panic! The dealer will not rebuild it. They will have to replace it. Most dealers will not rebuild these units. If the tranny is your problem, just be glad that you have that warrenty. Good luck!
