Torque converter locking mechanism is disengaging or engaging, causing a slight jerk at cruising speeds when there is no gear change. If you catch the RPM's with your eyes when it happens, it slows down or speeds up about 200-400 rpm based on weather you are accelerating or decelerating.
Mine's a little weird, it usually happens when I was holding the throttle down to maintain a highway speed and let go to slow down a few MPH and I press it back down a bit, mine can get confused and disengage and re-engage itself almost immediately 3 times in a row, making it seem like my gears are stripped to hell. (I know they aren’t, I can't cause them to jerk under acceleration or downshift induced deceleration) This normally happens after I've been driving for at least 50 miles and the torque converter and gear fluids have warmed up significantly. IE: much more in the summer.
If the RPM's change a significant amount, it's a gear shift. Otherwise it's the locking mechanism... Or a tooth on one of your gears is missing, it just slipped and your screwed out of at least $2,000 :P
Without the locking mechanism, our cars would have drastically crappier gas mileage on the highways since when it is disengaged, the only connection the engine has to the rear wheels is though the viscus fluid in the turbines, this is why automatic tranny's suck.
Normal operation and I wouldn't worry about it unless it STARTS getting worse. If it starts getting noticeable worse, then there IS a problem. If you want to have a level of security without disassembling the gearbox, change the fluids and examine the sump magnet and filter for metal shavings. That's all you can do without blowing at least $500 on a mechanic to drop the transmission and crack it open to take a look, and if you do that, there's a 50-50 chance that it will never run the same again.