Bubbles on the dipstick? Does it feel like there is some water in the tranny oil? Most automatic cars have a tranny oil cooler in the radiator, make sure the oil cooler isn't leaking and mixing up the tranny oil and water (if your's has one). And as Sydahsan said, change plugs, and other related things before going for the tranny.
AND since your tranny is shifting just fine.. and not letting the power go to the wheels as it should, check the water thing I told you.. have your trusted mechanic look at the dipstick and see what he says..
after all that, since your oil colour is brownish, id go for what sydahsan said.
And umm, how does the engine sound when the car is in Neutral? Does it rev fine? sounds fine?