Scary problem when driving on a very hilly, very treacherous highway. Hot day, had been on highway about 1 1/2 hr.We were coming up a steep section caught behind other cars so going slow. Got to the crest of the hill and was coming down steep section when the car sputters, lags and dies. No breaks, power stearing, etc. Luckily knew what to do and luckily there was a pulloff close. Car would not start - no gas getting through. Would start and sputter and die within 5-10 minutes. Let it sit 20 min and drove for about 5 minutes, hit another hill and once again sputter, rev., sputter - no power and die. Let sit for 1/2 hr. got cooler temps. and flatter road and drove last 3 hrs home no problems. Car did this once before in town. had been driving for a while, up big hill, hot day, let idle while waiting 10 min. Go top drive off - same symptons. Got car checked out - nothing. So had spark plugs, distributor cap, filters, etc. changed. Car occasionally lagged a little in between then and this occurrance. Car in at Volvo now and can't find anything that jumps out. It is not the fuel pump relay. Someone help. What could this be. The car is not getting gas through when this happens