2001 loss of power while driving

I bought my 2001 Prius used about a year and a half ago and had no problems until the past couple of months. On three occasions now, the triangle warning light has come up on the screen while I was driving and the car wouldn't accelerate when I pushed the gas pedal. Fortunately, the I was able to coast to the side of the road on each occasion, and when I turned the car off and restarted it, it worked fine. The first two times, it happened on the freeway after a lengthy drive and while traveling at about 70 mph. Today it happened on a surface street, but after I had been on the freeway (in gridlock) for nearly an hour.

I should have dealt with this sooner, but somehow every time it has happened, it was on the weekend and/or when I was out of town, so I couldn't immediately take it to a shop, and since the car started back up immediately and ran fine after each occurence, it didn't seem urgent.

I am going to take it to a dealer or Prius mechanic ASAP now, but can someone give some idea of common causes for this problem? I have seen some posts about similar problems related to gas pedal sensor issues. Is that the most likely cause, or are there other possible issues here?