I have a 2002 VW Jetta 1.8T (85K miles)that intermittently become hard to turn the steering wheel at low speed, like turning into a parking place or driveway. Seems to come and go. Thought it was because the tires needed to be replaced, but that did not stop the problem.
Possibilites come to mind.
Low power steering fluid (the leak will often be the rack seals).
Weak/failing power steering pump, either internally or at its bearings (see: binding).
Weak for failing rack piston seals.
Possible failing spool valve (part of the rack assembly)
Lastly, power steering pump belt slipping under load. If it’s driven by a serpentine belt, add a possible weak tensioner…allowing slippage under load.
In addition to a slipping power steering pump belt because of low tightness, defective tensioner, or old belt, consider water getting on the belt and pulley. If any of the under engine covers are loose, torn, or missing, road water can get onto the belt and/or pulley causing the pump pulley to slip under high load as at low speed and during parking.