It seems like the engine is misfiring?

Bought car brand new in 2004 - never a problem. Drives fine, then I drove 20 miles 2 days ago, on freeway- engine does a jerk feeling. Almost like the car is slipping on ice. Then drives fine for another 20 miles, does it again. Then miles later, does it again. Starts out slight, then get harder jerk feeling, only last about 3 seconds. No change in performance, no noise. Yesterday, took it for oil change, & replace fuel filter & a fuel system clean. Drove fine. Today after driving more than 50 miles, engine on freeway - does a jerk again. Just one time. Not while trying to acelerate. Doesnt feel like transmission slipping, no dragging - just a jerk feel - like the car is slipping on ice for a few seconds. But its not ice. Could it be a sensor going bad? Check engine light does not come on or any system messages display.

OK - 2 days ago, got a tune up at a lic. Facility - Platinum plugs and wires, - their computer dia - showed nothing - then the car jerked 4 times on the way home. At around 38-40 mph the rpm doesnt spike, just dips slightly or not at all. A transmission place stated this is a common issue with a 04 Ford taurus and recommends a complete re-build to fix seals - cost of $2,200. Then drove it yesterday over 100 miles - didnt jerk. Any further ideas? Thanks