JD 755 Rock Shaft/3 point Hitch Pulsing

[rarmstrong61]

My ~35 year old tractor started have an issue with rockshaft/3 pt hitch pulsing ( up and down <5% of travel at a frequency of about 2 times per second when loaded ( chipper attached or me standing on the 3pt hitch) and not moving up or down. I changed the Hydraulic oil, filter and checked strainer and re filled but still doing the same thing. It drifts down faster than it used to when stopped. I pulled apart the 2 valves I could find...the one next to the adjustment knob looked very worn...circumferential grooving...looks like erosion. I tried running it with that valve and spring removed and it pulsed even more....I think the problem is pretty obvious but I don't know how to fix it and am not sure if this is a sign that the tractor has reached the end if its practical life since John Deere no longer supports it with parts. The valve with the plug in the center of this attached photo....right above the port going to the adjustment knob. Advice?