I would take it to another dealer also. There is a reason for your problems. There are also procedures to follow for diagnostics. If there aren't any codes, there are symptoms to work from. Sounds to me like present dealer's service department is trying to take the easy stuff and make excuses for what they don't check. I had a dealer like that when I had my 05 PSD SD F250. Finally got rid of truck and dealer also. After I called that dealership and explained why I wasn't interested in their mailings and won't buy from them again, the service department got a house cleaning. I still won't go back to that dealer.