Back when I was a teenager on the farm, I had a hydraulic valve stick on our tractor just from heat. It was a very hot day and I was cultivating, and I was making several very short back and forth swaths in one particular area where I had to lift the cultivator up (then drop it again) several times in succession, only a few seconds between operations. Towards the end of the dinking around, the control lever started to become more and more difficult to move, almost to the point of totally seizing. Once I got out into the open again and the valve had time to cool down somewhat, it was back to normal. Never happened before that and never happened again after. Could it have been debris? Possible, but it didn't seem to fit the cause as well as over heating did.