1999 Nissan Maxima About the transmission

Hey, I have a quick question... Usually not all the time when I start my car and drive it to go from place to place, it starts to kick....the rpm i see it tap up n down....what could be the issue of this...when i first bought this vehicle i went and showed it to the dealer. But at that time it didnt kick, so he couldn't do anything about it. BUt now its doing it more than ever..Also, my service engine light turned on and off a few times last night when I was gasing it to go ont he freeway...Please help me out, thank you...