intermittent loss of power steering

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