Popping out of gear....
1) You have bad synchros.
2) Parts were incorrectly installed.
3) Improperly installed locking balls(detents).
4) Bad Hubs
You said you have it rebuilt, so I doubt number one is the cause. I contacted Joliroger about a problem I had since he rebuilt many R154s and he told me, if it goes back together improperly, they just won't go into gear at all.
Bad hubs could be a possibility. I made sure when I rebuilt my transmission, I checked that the locking tabs for each hubs were installed properly. The little tabs for each hub have a notch on both ends which holds the spring. If that is installed improperly, your transmission would not even shift correctly.
This leaves the detents. They are locking mechanisms that hold each rod in place when in and out of gear. That can be worn or the rod. You could try changing the locking balls and springs but if it is the oval shaped ones, you will need to take your transmission apart as those drop down from the top of the intermediate plate after placing a new rod in place.
