If you can both scan and physically inspect your cars transmission you don't need a mechanic first.
Seriously now, you could have a few issues, the tranny could be shot or, fluid issues, pressure issues, electrical and so on. However even if shot a decent rebuild can be had and you can be on the road again in no time.
Years ago my 95 325i auto tranny went and I found one for $900 that was recently rebuilt, install cost was not a factor for me but expect to spend $400+/- since fluid is needed etc.
So get to a shop and know in short order where you stand.
Also what do you mean the revs change, without stepping on the gas they jump while your foot is on the brake and you change gears?