Yard Machine 20 HP LT jerks while in motion?

This problem has been gradually getting worse. The faster I press the gas, the more my rider jerks. It's a constant jerk unless I go very, very slowly. We've had this for 7 years and it's been pretty much a lemon. We are looking at buying a new one, but if there is a fix that isn't that tough, I'd like to know.
Thanks,