1997 Sable Power Steering locking up

97 sable 106K 3.0 SOHC, the steering randomly loses power assist and then after changing gears (Drive to Reverse or whatever) or shutting the car off and waiting for about 20 seconds gets it back and there is no problem, fluid is topped off, the system has never been touched since the car is new, no unusual noises belt is good and appropriately tensioned pump is spinning freely any ideas, im thinking a pressure switch is going bad or something i havent examined the vehicle yet its too cold and snowy out