On rare occasion at highway speed, steering.

On rare occasion at highway speed, steering wheel begins to oscillate (not vibrate). It rapidly becomes severe in intensity. Steering seems not to be affected; but tremor rapidly becomes uncontrollable as though something is very, very loose or disconnected. Slowing down only slows down the intensity (not subject to specific speed as in balance). After car sits for a period as in overnight, car drives normally. After that, next event is unpredictable--maybe today or next month. Nothing is apparently broken. Mechanic suggests entire rack and pinion assembly. Does Cadillac know about this flaw and its likely consequence? It feels like a wheel is loosening and about to actually fall off. Annie Walker