1999 Volkswagen Beetle Engine jerking

I have a '99 VW beetle that I just bought from a co-worker who told me "make sure you warm it before you take off because it'll go crazy". Every morning since I have been warming up the car for at least 20 minutes before going anywhere and when I take off it jerks a little going into second gear mostly, but then goes away. However, just recently I warmed it up as usual, tried to take off and it began jerking pretty violently. It felt like an automotive seizure! I tried going into second and third gear but the jerking continued. I also tried giving it gas and it basically didn't pick up the gear. It also did the same thing when I didn't warm up the car. Sometimes, I can smell a gasoline-like odor and it'll make a low "put-put"sound that I can hear under the car. Help?