Height adjuster moves 355D

[Freetle]

Hi, I have an aging JD 355D. The height adjustment knob has started wandering about when the deck is raised, not funny when you set it at 4 (high) and lower the deck only to find that it has dropped to 1/2 and the blades hit the ground. It is locked and not adjustable when the deck is down as it should be. Any help would be appreciated.