UPDATE!

I went to the car to do some more testing. I went ahead to bleed the power steering system.. no changes. Steering working fine. After letting it idle for about 15-20 mins I revved the engine and noticed the whine sound didn't fade away around 14-500rpm as per before, it stayed LOUD until around 3000rpm... when I let off the gas pedal the engine almost stalled THEN the power steering was completely gone, this time it didnt come back.. no power steering and a very loud whine when revving. I guess I need to look for a new pump?

BUT can someone tell me why the power steering only stops working after the engine almost stalls out?

 

Another update. Now it’s working when the engine is revved but not at idle?

 

Drag on pump can lower the engine rpms .Belt can be slipping because of drag especially at lower revs when loaded more .

 

It’s completely gone now, no power steering at all. The pump is making a loud whine.. I tried to bleed the system several times more, fluid pours out of the reservoir occasionally.

 

Im not familiar with Navigators,but does the pressure line that runs to the pump have any type of screen/filter on the end of it or in the pump inlet hole where the hose connects to the pump?
If it does and if the filter is partially clogged with debris,it could be slowing down/preventing flow through the pump and hoses.Some power steering pumps/hoses also have a switch mounted in them that signals when extra rpm is required from the load of the pump by triggering the iac valve to open which increases idle,to prevent the engine from idling too low or stalling.However,from the symptoms you listed,it sounds more like restricted (from the fluid pouring out of the reservoir) flow issue or a suspect pump.Where the pump is getting flow one minute then loosing it the next minute.When its in the act of not working,pull the belt off the pulley and try spinning it by hand.A good pump should have some resistance,but not too much to where you have to strain to spin the pulley.

 

Just to add to good advice given above , not sure of your vehicle set up but I remember some BMW s and Jags to name a few , had replaceable reservoir filters . Some others had internal screens that through time got clogged up .Might not apply and could be the pump itself that's goosed but worth a look .