Ignition problems of the 1999 Chevrolet Express

While driving at a speed of 65mph on the highway, my van has completely lost all operation. My car quits running, steering freezes up and I lose my brakes. The car completely freeze up. I have had it towed twice in 2 weeks for this condition. The mechanic said a fuse blows in the computer. They cannot find a short or any reason for the problem. I am afraid to drive the car on the highway now. I never know when it will completely die while driving. What can it be? the mechanic ran diagnostic testing and thinks it could be something called ecm. But he is not sure. What do I need to do?.