Intermittent Hard Steering

[grfsSHO]

...specifically at highway speeds ('98 SHO / 178K miles). This is not the usual increase in steering firmness associated with VAPS, but very stiff, intermittent, steering with almost no self-centering on the highway. Low speed driving has normal assist. My PS maintenance is done ever 6 months with Mobil 1 ATF, so no issue there. Alignment is fine, plus I've replaced both tired tie-rod ends and one of the inners.
What are the signs/symptoms of a warn steering rack?

 

[stephen newberg]

That is not one of them, so I doubt it is the steering rack.

You may have done all the right things for the power steering, but it really does sound like what is happening is that the power steering system is cutting in and out for some reason or another. With that kind of characteristic, I would be tempted to say it is going to be an electrical fault somewhere in the power steering circuitry.

pax, smn