The engine rpm may be dropping too low to drive the power steering pump fast enough. This could be because the engine is out of tune, or the power steering pressure switch isn’t working, or etc.
Drive in large circles (in a large parking lot, cul de sac, etc.). Pay attention to the engine rpm (you could have a passenger do the observing). Does the rpm drop below the usual idle rpm? [You can tell by SOUND, if you don’t have an rpm gauge.]