Instrument panel goes black

Intermittent problem when the temperature is ove 80 degrees (it seems). The car will hic-up, feels like I hit a pot-hole, once or twice or more. Then the instrument panel goes black. Sometimes ABS and air bag warning lights come on briefly before it goes black. The car continues to run just fine, but the signal & brake lights don't work and windows don't go up. Happened for the first time a year ago - I freaked and had it towed, mechanic found nothing the next day. Took it to Volvo dealer, the upgraded the computer, otherwise no idea. But it keeps happening intermittendly, twice last summer - June and August, now twice alread this year, usually on a shopping trip, i.E. Driving short disances, parking, driving again. Last time it happened I parked for 1/2 hour and everything came back on and I drove home, except SRS light stays on, has to be reset. Why is this happening? I'm afraid to drive this car I love. HELP