You may very well not need a new transmission or rebuilt.
Some of the things to check before tearing it down.
1) Have you checked the oil level. It does not take much under filling for the transmission to behave erratically.
2) When was the last time the oil/filter service been done.
3) Is the transmission modulator operating as it should.
4) Any problem with the linkage (shifter)?
5) Is the cable from the throttle linkage adjusted properly ?
Keep us posted.