Violent jerk

My vehicle jerks violently one time at about thirty mph then goes back to normal. Sometimes it does it sometimes it does not. But most of the time it does it. Only when I am holding the pedal down or coasting and reach thirty mph. If I tap the peddle until I get past thirty mph it does not jerk. The code for the throttle position sensor popped up when checked. Transmission fluid is also dark. Could the sensor be the problem or could the transmission be going bad? A mechanic stated that the team may be going bad but I do not think so. I need another opinion because I think he is just trying to get more money for putting in a new transmission.