Glad I asked for a picture!
Well real simple your trying to do too much with the lift!
The three point on that model is rated at 400lbs at the end of the lift arms, and goes down as you move away from the tractor.
It's over loaded, by how much I couldn't tell you, but it's over.
Simple test to see if you have any issues at all with the lift, put 400lbs on a bar or plank on top of the three point arms and see if it will lift that, if it does then it's fine if it doesn't then we can look at other issues.
And no changing the pump will not give you more lifting power in this case.