Whenever the car is stop and idling at 600 rpm,
the steering wheel is very hard to turn.
I have to rev the engine a bit to turn.
Once car is rolling, turning is OK.
When I set the idling around 1000 rpm,
steering is OK.
An underdrive pulley is usually comprised of a smaller-than-normal crank pulley, a larger-than-normal waterpump and alternator pulley, and sometimes a larger-than-normal power steering pulley. All of the resizing is an attempt to free up horsepower that was being eaten by engine accessories, especially at higher RPM. This is accomplished by actually slowing down the accessories in relation to engine speed.
I was thinking that if you had an underdrive pulley set, it might push your power steering pump below the working threshold when idling at 600 RPM.
Or, it could just be a bad pump. The simpler answer is usually the correct one anyway.