'87 Ford Taurus shutting off randomly

My dad has an ’87 Ford Taurus (automatic) with about 90,000 miles on it. A few months ago it started shutting off while driving. In the beginning it would stall at an intersection and then start right up. He took it to the mechanic and had it fixed. Shortly after, it started turning off while driving (as if you turned the key off). It doesn’t matter how fast you are going. 50 or 15 mph it doesn’t care. If you pop it into neutral it will start right up. If you leave it in gear, it locks and you are stuck. This happened erratically. It will be fine for a week and then one day it might happen 6 or 7 times. He took it to a mechanic and was told that it was the transmission. He then took it to a transmission shop and was told that there was absolutely no way it was the transmission. It had to be something electrical. Going back to the mechanic, he was again told it was the transmission. The big problem is that it will never do it when you want it to (big surprise). We even left it with the mechanic for a week. So neither the mechanic nor the transmission guy has seen it in action. Do you have any suggestions? Thanks.