Check the tightness of the +12V power feed and ground cable of the power rack. Also check the rack fuse connections for tightness. The fuse is located at the front of the underhood fuse/relay box, under the front of the black plastic cover, attached to the metal bus-bar.
The electric power steering rack might have an internal intermittent issue...there have been a few of these reported in the past. Unfortunatly the entire rack must be replaced, the electronic control module portion is cannot be purchased by itself.