1992 Pontiac Bonneville Sometimes no first or reverse

This trans was rebuilt a few years ago. Now when I go to put it into gear, most of the time it is fine, sometimes however it will refuse to go into gear (first or reverse). Shifting between L and R a few times seems to get it going, particularly in the morning.

In addition it seems like if I stop a little faster than normal it goes out of gear. Sometimes it will come right back into gear, sometimes I have to move the shifter into L to get it back into gear. Every once in a while this happens when I stop normally.

The rest of the time the trans is fine, shifting properly. I do a lot of highway driving and it seems like the shifts are strong and accurate as they should be. Downshifting on passing is as expected.

Fluid level is good, color is excellent, no weird smells. No check engine lights, and the car is well maintained.