Why does my 2003 Pontiac Grand Prix low oil level light and change oil soon light come on at the same time? Then the car dies.

I have a 2003 Pontiac Grand Prix, and I was driving it in Minnesota on Saturday
8/1/2015.  The weather was in the 80's.  My change oil soon and low oil level light cam
on at the same time, and the RPMs went down.  Then, they would go off, and come
back on a few seconds later.  After it did this about 3-4 times.  It would then just
completely die while driving it.  I would lose power steering and stop.  It would then
start back up right away.  It continued to do this over and over again the whole, but
would start right back up again.  I then took it to the repair shop on Monday 8/3/2015,
and that cannot find anything wrong with it.  No codes were tripped, and it will not tell
them why it stalled.