2006 Ford Focus Jerky Gearbox

I'm driving a 1600cc Ford Focus & the automatic gearbox tends to jerk when it's at speed of 20km mark (which is at 2nd gear). This is especially so when I'm doing an acceleration (i.e. when negiotiating a bend, i slow down the car to about 20km & when traffic is cleared, the moment I step on the gas pedal, the jerk occurs). Sometimes, I can hear a "thud" sound. Have send my car to Authorized Distributor for check but was told there's no fault code. Need advise on this matter.