audi 100 5cyl Automatic transmission problem.

I have a '91 audi 100 5cyl 2.3E. when driving it changes to second fine and to third fine but in third when it tries to change down again the revs drop off 100 or so then it changes but it seems like it just changed back into 3rd because the revs are the same as before with the same power, after that it changes into final drive fine without problem. It only does this once in third gear until its changed up or down, then it will do it again when its in third. any ideas what could be wrong?