Cat 312 bl, jerky hydraulics

[EH1]

I have a cat 312bl,98 mod. It has 12500h on the clock. I was bought only a few years ago as a farm machine and i have only used it a few houndred hours of the total. Right from the start i noticed that the hydraulic became jerky when the fluid became warm. When operating and tracking at the same time the tracks would become jumpy and i had to let go of the pedals and carefully try again and then it would be all right. Some times i can sense the same jerky movement when only operating boom and stick . I also think the engine is running warmer than before, sometimes all the way to red only after mabe 10 minutes of tracking. The swivel is recently resealed as it tracked off line before, but now it goes straight.