2004 srx awd 6 cy

Have experienced the following 3 times the dealer 1 time. At speeds of 70 to 75 mph the engine will shut down to approx. 20 to 25 mph immediately. The resulting confusion to the motorists following is very interesting and damm scary. GM is having problems diagnosing the problem as the computer does not register any faults. The car has been in the garage for some 30 days. So far they have replaced the throttle body, a clip, a throttle rod, and finally now a complete wiring harness. There is no guarantee that any of this will either eliminate the problem or not. At this point in time I have no confidence in the car. HELP