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I would guess either the pump itself has failed or you have a clog in one of your lines going into the rack the silver ones ! Maybe the rack seal blew out internally! No power steering isn’t really hazardous but in this situation of it coming and going when it feels def don’t drive it until it’s fixed !
Have you checked to see if the harmonic balancer outer ring is slipping on the inner hub??? Paint a white line on the front of the dampener. Over the outer ring, over the rubber and on to the hub. Take it for a drive and make it fail and then look to see if that painted line on the front of the dampener is displaced. Very common issue.
Also check to see if you have a properly working belt tensioner and the pully on the PS Pump isn't slipping on the pump shaft.
Do the same painted line on the PS pump shaft and PS pully and see if it shifts after you get a loss of steering assist.