why would my pontiac grand am stall while riding down the highway after driving a

while? i am at a loss? we have replaced everything that suggested by the mechanic and it still stalls out while driving on the interstate. we replaced the 02 sensor, fuel filter, plugs, some sort of idle valve and it still just looses all power and rolls to a stop and after sitting a minute starts right back up and goes for another 100 or so miles. someone please help!