2003 Chevy Silverado Engine boggs down at 2600 RPM

I resently bought a 2003 duramax after the first couple of months the MPG gallon dropped four miles a gallon and the engine will not respond above 2600 RPM if the torque converter is locked in fith gear the truck will down shift to fourth gear and the torque converter will stayed locked and the vehicle will not accelerate. Any idea?

Here is what I found; new low sulfur diesel has a tendency to allow bacteria to grow, I added an anti-bacteria solution to my fuel and replaced the fuel filter and the truck has return to normal.

PS After reading the Duramax diesel book I found for the vehicle there is even a note that states: a loss of mileage, hard starts, hesitation during hard acceleration and flat line performance at high rpm is an initial indication of a clogged fuel filter. Thanks!