It sounds to me like your safety on the loader valve is misadjusted or stuck. The valves are pretty simple. The main system flow is thru the loader valve to the diverter valve down by your right foot, that diverts oil from the 3PH to steering when you turn the wheel(steering takes priority over 3PH operation). There is always flow in the system. The flow thru the loader valve is re-directed from it's path to the rest of the system and sent to a cylinder when a control lever/joystick is moved. The fluid returning to the loader valve from the other side of the moveing cylinders is sent on to the rest of the hydraulic system. The fluid must always flow so if the loader has too great a load, the safety valve in the loader will open and fluid will bypass the cylinders and go on through the system.
The clue about it working when cold and when turning the wheel or lifting the 3PH once the fluid is hot is interesting. When those systems are used(work is being performed), pressure is placed on the system all the way back to the pump(including the loader valve). If the safety valve in the loader is set too low or is stuck open, cold fluid would not flow thru a partially open safety valve as easilly as warm fluid so when cold, there would be some resistance here and the loader would work but probably not to it's full capacity. As the fluid warms, it flows easier thru the safety and you can't perform any work with the loader. A little back pressure from the steering or 3PH and you again regain some loader ability.
The first place I would look is the safety valve on the loader. Then either get a 0-5000 PSI hydraulic gauge($12 an Northern) and connect it to one of the working ports(one of the lines that goes to a cylinder). To check the pressure that the safety is releasing at or take it to a hydraulic mechanic to work on. Depending on the make of the valve, yo may also need adapters to match a SAE gauge to a metric port.
To check the safety pressure, you start the engine and send pressure to the port with the gauge. Since there will be no flow, the safety will have to open to maintain flow and the gauge will show the pressure at which this is happening. Be sure you move the valve the right way as if you send pressure to the other working port, the cylinders will move and oil returning from the other side of those cylinders will squirt out of the hose you disconnected to attach the gauge. I usually place that hose end in a clean bucket just in case. If the safety release is not set correct, you should be able to adjust it to the proper value. I have seen values for the 200 series between 2000 and 2300 PSI. I set mine for 2000 PSI. If I can't do the job with 2000 PSI, I am using the wrong tool.
The safety valve is a plate or plug held in place with a spring. When the pressure exceeds the spring tension, the valve opens. Some are set with a screw and lock nut that sets tension on the spring or some(such as the safety valve in the 3PH) use washers as shims to set the spring tension. Before wrenching on any hydraulic system, make sure everything is on the ground, nothing is running and no pressure is left in the system(work all the valve controls a few times). Wear safety glasses.
If you think this is beyond your abilities, you could de-install the valve(see above safety precautions
and take it to a hydraulic shop so a mechanic can check it out, take the whole tractor to a shop or have a mechanic come to you. Most shops should be able to test the valve function and set the safety.
