My AW70 does the same thing. It shifts at 10, 15, and 30-32. If I'm not accelerating much. I have to be really pushing it to stay in second gear for long. My car has 232K on it. I had the trans fluid changed for me last year, but had dino(saur) fluid as a replacement. I think this summer, I'll go to synthetic and do it myself. Maybe a guru will post an answer about this. I think that perhaps my trans needs a valve body service. I personally like the way it shifts. Why not stay at low rpms if higher rpms aren't necessary?
The thing that annoys me most about newer cars (like my girlfriend's 98 dodge neon) is how they shift at preset road speeds, not trans pressure.

-Will
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1990 740 Turbo, on its way to stock specs, maybe beyond