2003 Sable stops at hwy speeds when hot outside

This 2003 Sable with the std engine has less than 60K miles. The engine is stumbling and quitting while slowing at highway speeds. The car will start after a brief time. This only happens when it is hot outside and is intermittent, does not happen every day. It is showing a code P0191, but the engine light does not stay on. The car has been to the dealer twice and they can find nothing wrong with the car.

Problem is intermittent and probably this can be solved by replacing parts until the correct one is found. The fuel rail sensor has already been replaced.

Any idea what part should be replaced next?