mine used to do it, but after much research...i found a solution. I drained my tranny fluid...Bought one(1) quart of 5w30 motor oil, then bought one(1) quart of GM sychromesh (from GM dealer), also bought one(1) Lucas oil stabilizer(from autozone)...
~started by adding the one quart of motor oil in tranny, then 1/4 of oil stabilizer(be patient cause its really thick), then add the one quart of the gm sychromesh, then another 1/4 of oil stabilizer..
~ took it on the freeway, and it popped out once, then after that has not popped out since... I hope this is useful to you, I thought i was going to have to replace the tranny, but after researching alot, found out that it was only a $34.00 problem..