Help, if you can!

My 90 Carat with 106K automatic trans is starting to act peculiar.  I
had it serviced (fluid and screens cleaned), but no help.  Symptoms are
when shifting from 2nd to 3rd on low level of acceleration, seems to
shift to 3rd,  then release and apparently head back to 2nd.  Engine
revs like it is almost in neutual.  After a few seconds, it goes back to
3rd and seems OK.  Under hard acceleration, it shifts as expected/as is
should.  No other funny noises.  Has anyone else had this?  Am I looking
at a rebuild, God forbid?!  Thanks in advance,

Ed
90 Carat 106K