2002 Toyota Prius Idiot Lights Fluctuating

I recently acquired a used 2002 Prius with 95000 miles on the odometer but which had a complete engine and battery pack replacement under extended warranty 17000 miles ago. Service records indicate it's been faithfully maintaned by a Toyota dealer since it was bought new in 2002. It drives perfectly. However, while driving, from time to time all of the idiot lights start to glow from very dim to about half their brightest level. If I step on the brakes the lights get dimmer and finally go off completely. Sometimes this recurs during the same trip, but generally doesn't happen again until the car has been parked for few hours. My question: is this normal, or does it signal a problem? I can't find any mention of this issue in the owner's manual. The only reference is to warning lights that come on and stay on, requiring service. Should I be worrying about an impending major repair?