My 940 has a problem that my mechanic cannot identify. I can be driving (usually at lower speeds) and the gas pedal just seems to feed nothing to the enjine. Eventually all lights come and the car enjine is dead. After a minute or two it will start, usually though, not on the first try.
Try another mechanic that can diagnose these types of problems. You are getting nowhere at high speed.
My first guess would be to look at the fuel system as the source of the trouble. Maybe something like a faulty fuel pump relay is cutting the power to the pump.