machine becomes very jerky when the hydraulics comes up to operating temp.Also has had a hissing sound in the valve stack for very long time-even had it in a cat shop numerous times but no fix. finally replaced part number 423-4562 and hissing went away.Old part was subbed to part 601 4559. Jerkiness and hissing went away for about 8 hrs operating time and now jerkiness is back but no hissing.Also valve gets very hot to touch. thoughts and comments? thanks