We do need more information, but the first step is to check the fluid level. Next look at the color of the fluid. Is it black, clear, or milky? If it is milky or black, change it. If the tractor was new to me, I would change the Hyd filter and oil just as preventive maintenance. It is easiest to troubleshoot the systems by steps. If the oil is good, the first step for me would be a pressure test at the pump while the problem is happening. There are a few things that could cause the pressure to drop. Bad pump, clogged filter, failed safety pressure release valve. This would be a place to start. Let us know what happens.
Intermittent weak lift make me think clogged filter, 3 point hitch control valve, or safety pressure release valve. Usually if a pump goes out, it goes out. I have never seen one get better own its own.