Years ago, I had a WD that the previous owner hooked a chain on each side and then looped it over the axel. When he wanted to change height, he just moved the hook up or down a link. It worked extremely well. I have a picture that shows it quite well. Send me an email and I'll send it to you.
I have a WD and a WD-45 now. Neither of them hold perfectly but adjustment is like once every half hour. I think when your tractor was new it probably held well. Now, the valve is probably worn. I've had these valves apart but never rebuilt one. Hopefully some other posting person has had this experience.
I am assuming you realize that the valve can be set up for two different operations. One is lift and the other is lift and hold. If it can't be held in a position for several minutes, You may not have the valve set to lift and hold. The WD operations manual explains the adjustment very well. If you don't have one, it is a good investment.