Yeah usually when they act up after warming up there is a pressure problem, when the fluid comes up to full temp it's too thin to maintain line pressure through the whole hydraulic circuit. Can be from a weak pump, blown accumulator, seals bleeding out, etc...at any rate it's usually big repair, unless pressure test show that one easy to access component is the cause...which is never the case for me....
...and usually if it's been running for some time with low pressure everything in it has been overheated and overworked, it's a major failure waiting to happen.