01 Sonata problems

I've had an 01 Sonata for about a year now .. no problems until about a month or two ago. I'll step on the gas and it takes a second to kick in and then all of a sudden it "catches" and will lurch forward and drive. It happens in the morning when I first drive the car.. after I stop at red lights, etc. It never seems to let up. Sometimes, it gets real bad and I have to pull over and turn the car off and then on again or else it will kick in as I am driving and my car kinda thuds as it lurches forward.

Shortly before this problem, my check engine light came on but a few days later it went off. I had my transmission fluid completely pushed out and changed ...

any ideas? I'm not sure if I am still under Hyundai warranty as I bought this used not directly from a pre-owned Hyundai dealership.
thanks in advance for any help you can provide me.