I'm no cummins expert but I do mess around with them some. First I would install a fuel press guage and see what you have, if it's low then your lift pump is going out. Second I would check the transfer pump to make sure it's not a dud. I would also reccomend that you go to a cummins forum like this one www.dieseltruckresource.com .
I'm on there and there is a wealth of info on our dodge trucks.