Stalling/shut down while driving

I just bought this car and all seemed well, but now after four days of ownership the car keeps shutting down while I am driving on the highway! Very dangerous and scary at high speeds. The first time it happened with no previous warning. The next few times it would begin to power down then come back to full operation then shut off a few minutes later. After it stalled out I could disconnect the battery or pull the main relay and it will usually fire back up after ten or so minutes sometimes even right away. But then will either drive a few blocks or a few day then do it again. My mechanic ran the code which I forgot to write down and which has disappeared since my last reset, but it is a code for the speed input/output sensor on the transmission. As far as I know a sensor issue for the speed sensor would not cause this kind of shut down. My mechanic has tried driving it around and it will not stall while he is driving (of course), but it drives fine for me to unless on the highway. Due to the danger of possible accidents he has not taken it on the highway but has taken it up to decent speeds on some back roads hoping to recreate the issue to no avail. So after much reading we are leaning toward a crank positioning sensor but the Kia dealership says that if it is not throwing a code for the crank sensor that that is unlikely to be the issue. It is an expensive fix so I am wondering how one can check it as it is nearly impossible to get to to test while the car is running?