Occasionally my 2008 Pontiac Grand Prix 3.8 will just plain stop. The engine stops & the steering becomes very hard. I pull over, put the car in park & it starts right back up. I drive the care days or weeks with no problem & all of a sudden at speeds under 35 or so it seems the engine will stop again while driving. I thought bad gas but I've filled the tank many times since this first started & have added a fuel conditioner that is supposed to remove water in the gas in case this is the problem. I use a top tier gas medium grade. I can sometime drive the car three weeks with no problem and it will then suddenly just plain stop running while driving. It always seems to start when I try to restart it & then runs OK for a period of time until it happens again. It seems to do this more often when it is hotter outside though the car hardly ever goes past half way on the temperature gauge. Many times the car runs well even on warmer days. There is no check engine light & no codes associated with this problem. I had the codes read & there were no codes to report. The problem is not consistent with anything I can see. The car runs fine otherwise. Any ideas? HELP!