My truck shifts fine sometimes, short ,fast shifts, then sometimes it does what I call flare between shifts. When it "flares" the engine will climb 300-500 rpm between shifts, pedal never moving. No box or programmer installed at the moment.Is this normal? If I hook to a trailer it might do it or maybe not but no noticable slipping when driving. No SES light. Could it be valve body or clutch packs or torque converter?

 

I know this will sound funny, but try swapping out the fuel filter.

 

I just changed filter out and still does it.

 

It could be in the converter then. Is your oil level good and the color/condition of the fluid look okay ?

 

Level, color, and smell all ok. Mainly noticable in 1st - 3rd gear.

 

Sounds like you will need to go for a test drive with a scanner & watch what is happening when it acts up.
I don't think it's slipping, (unless it's in the converter) otherwise you would be hitting limp mode. Maybe a solenoid slightly acting up ?? (Should be codes then)

 

Might be your NSBU switch?