After 2 TH's and 15 years as an owner, I have never heard of a hydro line getting plugged.
If you can gigback, then the lines shouldn't be plugged. Has it worked at all since you got the machine? Sounds like some kind of issue surrounding the flow control valve. When you twist the valve, do you get any kind of reaction?
Just so I can sense whether you may have some sort of FrankenHarvester, when you say the lines jump when you push the knob, are you actually pushing some sort of knob or joystick or are you flipping a switch?
Normally, advance is accomplished by flipping a two position switch on the remote box into the up position and the feed speed is controlled by the hydraulic flow control valve, located on the arm near the turner/clamp etc. joysticks. (On older models, it may be on the head, near the engine and travels with the head.)
I don't mean to treat you like a dummy, just trying to get a feel for whether you have a stock factory mill or a hybrid of some sort.