1993 900S 4-door automatic.  135000 miles. My son tells me that on the way
home, the power steering went off for a few seconds and then came back on.
This happened to him several times in the space of a few minutes.  The fluid
is close to full. No warning lights came on.  Car didn't buck or anything
like that; engine was going smoothly. What could cause this intermittent
loss of power steering?
Thanks
Timo