I have a 2005 Ford F-350 with 40k miles. 6.0 v8 desiel automatic and the problem is that after 3 rpm it doesnt shift. you have to let go of the gas and then step on the gas then it will shift. i checked if there is any codes but it came back clear. transmition oil is good, air filter is good the turbo guage shows turbo is working, what can be the problem??