I think that you have a faulty coupler or tip at the backhoe supply hose. When the flow and or pressure increase to a certain point, it is pushing the ball in the coupler or tip into its seat blocking the flow. When you shut off the engine, your pressure drops, and the ball is then pushed off of its seat and the circuit is open again until you increase pressure or flow and it seats again.
Your hydraulic oil flows from the pump to the steering priority valve. It supplies steering demand and the rest goes to the backhoe or loop line if the hoe is detached. The oil then goes to the loader valve, then on to the three-point valve. If the coupler fails, stopping the flow, the pump is deadheaded and no oil can get to the loader or three-point. Your steering and PTO will probably still function.