Ive seen this problem many times with hydraulics, and after so many costly troubleshooting procedures, I have found what generally causes it is a bad hydraulic hose....
when we get a machine in that has the symptoms, after the pressures all check out good, we go directly to looking for a bad hose.... sometimes its just a failed seal somewhere in the system.
on hoses, when the fittings are installed, if they get over crimped, the hose liner will be damaged and a "flap" can develop on the inside of the hose at the end of the fitting on the inside, creating a flapper valve.... when it gets big enough it causes a restriction and sometimes it can cause a complete blockage, but the function always works smooth and proper in one direction, but in the other direction it can be either erratically or fail dependably..... and about the only way to find it is to systematically swap the hydraulic hoses in that circuit end for end, one at a time and test running it before swapping the next hose....
the other issue we sometimes find when this happens (usually on pilot operated valves), is the spool valve is sticking, due to a spool that has came loose and is unscrewing itself inside (depending on the design), a broken spring/adjustment came loose or a bit of contamination that is not letting it travel full in one direction....
there is little else that can cause the issue...