I have a 2005 Cummins Ram, & a few weeks ago after buying fuel at a new station the engine began to "blurble" at start up, as though it had water. So I opened the water drain valve, but that had no effect, did it numerous times with no luck. tried various additives & injector cleaners & at first it got better, then started again. Recently replaced the fuel filter with a new Baldwin (which I had previously done back in the winter, btw) but still no improvement. After the engine runs a few seconds it comes out of it, after I've opened the throttle. But it's annoying plus I don't want anything getting worse. What am I overlooking? Oh, and since that time it's had several fills of presumably top quality Shell diesel.

injector is going/gone.
start the truck when its cold and use your temp gun to shoot every exhaust ports on the manifold. you will find one or two are colder than the rest

that's at least where I would start

Slick!

Any white smoke when this happens?