1997 Lincoln Mark VIII Power Steering Problem

Dear Sirs, A couple of days ago, my steering began to stick, jerk, slip or have a stiff feeling when I turned the wheel? Fluid level is OK and the belt assembly were all OK. The only thing I can figure is that the Power steering pump may be going bad? There are no leaks. Are these symptoms typical of the pump malfunctioning? I read in the manual that you should never hold a hard right or left turn for a long time or you could damage the pump. Maybe over the years this may have happened and now the pump needs to be replaced. Have you heard of these symptoms before? Could there be any other problems. Also this car has speed sensitive steering. Thank You, John Peacock