Why did you need to add transmission fluid? Did you check the level with the engine running?
Have you had a mechanic read and record the diagnostic fault codes in the Transmission Computer? As a general rule, if the transmission starts out in first gear like normal, then bangs back to second gear during or right after a shift, slippage in a clutch pack is the first suspect. When it starts out in second gear as soon as you shift into "drive" after starting the engine, that slippage hasn't had a chance to occur yet. A failing speed sensor is a better suspect then because the problem was detected before any shifting occurred.
SPONSORED LINKS
Saturday, May 2nd, 2015 AT 9:35 PM