What is happening is the transmission is going through all four gears, then the torque converter locks up feeling like a "fifth" gear. This "fifth gear" will unlock as soon as it detects any significant load, and in fact has to unlock to allow the transmission to change down to third gear.

It sounds like what you are feeling is the torque converter "hunting", dropping in and out of lock. Given that you are towing a load then this behaviour would not be considered unusual

2002 Sporty , Ironman 2.5" spring, 2" body, 15 x 7 ROH wheels, K&N, 15 x 10.5 Simex Centipedes, Powerchip 91.

"Never argue with an idiot. They will bring you down to their level then beat you with experience!"