Like the title says, i have a first gen cummins in a 03 550. Stock other then turning the fuel pump up. Runs fine in the morning but when the outside temp gets up like in the afternoon it runs sluggish..the temp of the truck doesnt change at all but it idles about 150-200rpms less and it wont pull like it does in the morning. Anybody have any idea where to start looking?? Thanks!!