I have a 97 F350 powerstroke, the problem is at idle it will just shut off, it can run for a half hour or more and then just like someone turned off the key. I have changed the fuel filter and thought that fixed it but after driving for a while it shut off. Today started up just fine ran for 20 min then shut off. Started right up again ran for 20-30 min again then shut off. Any help ??