In my opinion, your drive motors are not big enough or you do not have a a deep enough reduction in your chain drives to get enough torque to turn the machine. It is not a too much pressure issue, more pressure actually helps you turn as it gives you more torque from your motors. If you are lugging/killing the engine, you are consuming too much power and need to regroup. Power consumption is a balancing act between pressure and flow and if you are having problems turning, it means you need to sacrifice flow in order to maintain your pressure. Pump size (i.e. flow output) has nothing to do with it's ability to turn.
Some numbers for comparison: Assuming your loader weighs around 1500 lbs, at a 2000 psi operating pressure you would need a 3 CID motor to turn the machine. There are a LOT of assumptions in this calculation but it might give you a place to start from where you are now. Notice that I don't mention flow in this, it make no difference in the turning ability of your machine, only the horsepower consumption and the speed of the machine.
Hope this helps.
MH49