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Plugged filter might cause it, and it might be blown o-rings in the standpipe, but the problem is consistent with a tired pump. You didn't mention jumpy behaviour. Blown valve o-rings in the pump cause loss of lift and jumping, but that's probably ruled out. As the oil warms up, it becomes thinner, and the pump simply cannot deliver enough. Filter is easy to check through the left inspection port (PTO lever mounted in it). Run the rear wheel up on a 6x6 to tip the oil away from the left side, so it doesn't run out when you crack the cover loose. Be sure to replace the safety wire when re-installing the filter! You can check for blown o-rings while the cover is off. With a load on the 3-point, block the tractor so it can't roll, start it at idle, lift the load, and with a light and mirror look for oil being driven under pressure out of the top of the pump or the standpipe ends. No leakage? tired pump. Overhaul time! Repair will involve removal of the pump - shop manual (MF or I&T) strongly recommended. They're available from our hosts here - top left of the page.
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