Yea, just in case... what tractor model, what kind of transmission... 4x4 or 4x2? is the tractor new to you or have you owned it for it's life? when does it jerk... can you associate it with anything? Basically we need more infor. - and that's for just a guess
Welcome. What does jerk mean. Does the engine miss or does it physically stutter. If the latter it may be a front differential gear problem assuming it is four wheel drive. Make, model, etc.?