Reasons why the PS System warning light comes on?

I have a 2001 Prius and have recently taken a road trip from San Diego to San jose in 100+ degree heat (real fun...). About 5 hours into the drive and a short time after traversing a steep incline/decline (called the Grapevine... Californians are aware of this...) all of a sudden the PS System Warning light appears. But the problem "corrects itself" (it says so on the screen) and goes away about 5 secs later... Five minutes after that the darn light comes on again... this time the problem doesnt go away and the steering becomes heavy odviously.... stop for gas at this point. Restart the vehicle and the PS warning light is nowhere to be seen... until about another hour later (and about 80 miles later). Again the light stays on and does not correct itself and stays on for the rest of my journey. The next day I start up the car and no PS warning light... havent seen it since but Im wondering if I will see it on my trip back home

The owners manual is VERY VAGUE about what the causes may be... hence Im writing to this forum to see if anybody has had this happen and what might be going on as it appears to be intermittent for sure. The constant here is that outside temp was 100+ degrees the whole trip if that even comes into play...

Thanks, Scott