Eng111Lt

I have a 2001 2500, 5.9 cummins. About a month ago i noticed my fuel pressure gauge on my pod was stuck on 5, then A few weeks ago i had a thermostat replaced in my truck. When i picked up my truck it would turn over then shut off. Then you can start it right back up and it will run. I checked the codes on the truck and it showed it was a throttle position sensor. I replaced the sensor and had the codes cleared. Its not throwing any codes but its still doing the same thing. What do i need to check next? Or what could the problem be? Thanks