Bongo Fury

hi all my freda has developed a bit of a problem. When i`m going up a gradient and i try to accelerate the revs hit 2.5 then waver as if it wants to change gear but cannot.Have to slow down then calmly gain speed but maybe one day i need speed quick. hope someone can help

Have you checked the auto fluid? when was it last changed? may need topping up/changing, possibly the filter inside needs a clean?