i have a 1999 chevy cargo express 1500 van 5.0L and it will shut off going down the road. sometimes it will crank back up and other times it takes awhile. it will turn over but won't stay cranked. I have replaced the TPS Sensor today and it still shuts off. Not sure if it is the fuel pump or the ignition switch. Any suggestions as to what it could be? Please help!