Is the pattern on the shift quadrant P R N D 2 1 or is it P R D 3 2 1? If it goes P R N D 2 1, you have an Aisin Warner transmission. If it goes P R N D 3 2 1, you have a ZF transmission.

On the surface, it sounds like a misaligned or loose shifter control.
Does the shifter handle feel like it has slop in it?
A possible explanation might be a worn bushing between the shifter & connecting rod not allowing the tranny to be fully shifted into gear allowing it to pop into reverse when it is placed into PARK.

Put the car up on jack stands.
See if the connecting rod between the shifter & tranny control lever is snug, no play at either connection point.

If it seems OK, check the alignment by disconnecting the rod and see if everything aligns when the tranny lever & shifter are both placed in the in the PARK position.

1. make sure you fluid level is correct once the transmission is fully warmed up.
incorrect fluid levels too much too little even more so can cause all sorts of nasty troubles.

2. if the fluid level is good and the fluid has NOT been fully flushed recently and is not a nice red color do full flush. read the faq's the procedure is easy and takes half hour in your driveway. verify you do not have metal bits or foreign matter sloshing around in the fluid

3. verify your transmission mount is in good order and not causing the transmission to sag out of alignment and also verify you shifter linkage is also properly installed

if after performing 1 2 and 3 no change for the better takes place you have a more serious problem which may require a replacement.