Likely stiction. I'm pretty sure my truck had it's first episode yesterday. These threads start to pop up more frequently as winter gets closer. My truck started great but for about 30 seconds it was a bit lumpy and I could see it on the tach. Cleared up by the time I reached the end of the driveway. More of a nuisance than a big issue. Syn oil and a product called Rev-X has been shown to make a good improvement.