2000 Mercury Marquis steering quickly getting tight

Rapidly increasing tight steering on my 2000 Merc Grand Marquis. Inspected all steering joints. Replaced Idler Arm. No change. Belt is tight and no leaks from power steering pump, fluid full and no groaning. Do not suspect Pitman arm joint but was thinking of replacing. Read some search results on the web. Apparently there exists a solonoid valve on the pump that is part of an EVO system that could be suspect. Also greasing joints along the steering column. The tightness is especially notable at rest , slow speeds. At highway speeds it steers straight but I feel the the resistance to steer. I do have to turn the steering wheel back manually at any speed.