My 93 Lexus LS 400 shut off when I was getting on the highway.

I was getting on the highway to go to work and turned on the air as I was driving down the on ramp.  My car just shut off.  I paid to get it towed to a repair shop.  They drove the car for 3 days and could not find anything wrong with it.  I then drove my car to work the next day and was entering the on ramp when my car shut off again.  The air was on again and had been from the begining on this trip.  Not sure if this is related..but just in case.