Usually that is an indication of a transmission that has worn clutches etc unfortunately. If you want to delay having to spend a lot of money it might pay to get the transmission serviced which may give it a few more thousand Ks of life. It may just be a clogged/dirty filter so once again get it serviced. If you are handy you can do the service yourself and save many $$$$.
I would also check the colour of the transmission fluid - it should be bright red....if its brown and has a burnt smell then it's Overdue for a service.
Next step is to throw in a can of 'auto trans fix'....there are different brands in all auto shops - this usually will make the clutches/bands grip better so that it delays the time for replacement.