it will be messy, but you should be able to disconnect the return line (plugging the reservoir so it doesnt leak) and see how much fluid is actually returning to the pump. have someone turn the wheel, lock to lock, and see what happens at the return hose. make sure you wear goggles, and have a bucket to catch the fluid in. you'll know right away, if it's the gearbox. you might also open the feed line, and see that it is pumping enough fluid as well, if the return seems weak. at least you'll know, where in the system the problem is. i kinda feel, the cooler is on the pump, for a reason myself. removing it will affect the temp.