I have a "modern" tractor, a JD 5303 2WD with front end loader, and I have a problem I can only describe as air in the hydraulic system.
Here's what's happening: At cold start up, everything is fine for say 5-10 minutes. Then I'll start noticing "hitches" in the steering, and also when lifting the front end loader. This will get progressively worse over the span of 2 hours (like when I'm cutting hay).
Let it sit for an hour or so, and it will be fine again.
The only thing I've done lately is top it off with Tractor Supply tranny/hydro fluid, yes I read the back and it said it was good for JD. But maybe this is the problem? I can't say if I had the problem before or after adding this fluid. You only really notice it after running a while.
I do not see anything spraying out like from a leak. I added the fluid only because we ran a leaky log splitter this winter (using tractor hydraulics), and lost some fluid that way.