I just bought a 2003 F 550 with 6.0 powerstroke.. I was driving it down the road it would only get going like 58 59 mph... got off the interstate it died on me.. sit there around 1 1/2 hrs then it restarted... started back on road then it started running 75 mph then just dies and wouldn't restart.. What could be the problem with this truck?. Don't want to just go buy expensive parts and start replacing.. Wondered if anyone had any problems like this and what they did to get it running again