Dash lights

Our car is a 2008 Prius. The dash lights intermittently do not go on when I start the car. When I drive the car a couple of miles the dash lights generally go on. The dimmer switch is always on at its highest level. This last time, however, the dash lights did not go on for two days. By the time I drive to the dealer about 15 miles away the lights came on. I have taken the car in to be checked a couple of times and the technicians can find nothing wrong. The service manager said they could do nothing as long as the lights are on. The problem is, as you can understand, the lights will always be on when I drive into the dealership. But it is worrisome that the dash lights from time to time do not come on. The ready light is on and the headlights/brake lights work and the car seems to run fine. But no dash lights for a few minutes. What to do?