Why does my transmission seem to lose power in drive?

What is your question?

My truck has the 5.9 diesel and the transmission was rebuilt into a heavy duty unit with an aftermarket torque converter. Two months out of warranty the drive gear seems to lose power at times. I have only put 14,000 miles on this transmission and had the fluid and filter changed when I started having problems. All other gears seem to be fine including overdrive. Could this be a sensor issue?