2001 Honda Civic stalls on highway above 70mph

Hello my civic ex has stalled or nearly stalled a few times on the highway when driving about 15 minutes at 70 mph or above. The Honda dealer mechanic is not sure what the problem is but said it might be a sensor. Nobody seems to know for sure. Could be a fuel pump relay? What is a mystery is it doesn't always happen and I noticed that if I drive below 60 mph it doesn't stall at all but if I drive 70 or more there is a good chance it will happen. When driving 10 minutes or so on the highway at 70+ all of the sudden the lights on the dash come on and the car is forced to slow down to a max speed of 30 mph and/or wants to shut down for 20 minutes