Here's a nice long discussion of agitation and uneven density in the film forum here-
http://www.apug.org/forums/forum37/8...-me-crazy.html

I don't know much about the Paterson plastic tanks. I do know that it took me a bit to get even development in sky areas with 120 film with stainless tanks and reels. There were two parts to the solution. One is to not overfill the tank. I measured out how much liquid I need to cover the reel, add about 1/4 inch, and then to get a round number for measurement. Leaving a large air space in the tank is critical to allow the solution room to actually move and flow.

The second element of the solution is to do fast movements. I turn the tank quickly,pause for a couple of seconds, and turn it back quickly. Again, the goal is to get good movement and flow across the face of the film.

The photographer Robert Adams ended up developing 120 in a darkroom by holding each end in one hand and sliding it through a large tray or tank. Like tray developing 4x5, but the long roll instead. I'm sure there is more to it than this, but he wanted completely even Colorado skies and tanks were simply not doing it.

I'm sure that Paterson tanks can work but others will have to tell you their methods.