when i first got my rocket it jumped out of first a few times and i was watching a thread about it.
what i would do is to make sure it went into first. i noticed that sometimes it went in soft (without clunk and it was more likely to happen then. so after a soft shift i would let the clutch out about half way to see if it would hold or not. some where along the way it quit doing that and has been a long time since it has.