Sounds like the notorious torque converter lock/unlock failure. Many of us here have had it, particularly with the 4.4 and 4.6. It manifests itself very much like a misfire about every 2 seconds under light acceleration. If it appears to go away in Sport mode or with the cruise control on, it's the torque converter. Both of these scenarios essentially lock up the TC so it doesn't keep unlocking. Search TC failure and symptoms on this site....there's a bunch of info. Brace for a TC replacement. I just drive in sport mode now and it's fine but a TC or full trans replacement is on the horizon. Best of luck.