Jumping out of first gear

[FireRescueFL]

Every now and then, my transmission will pop out of first gear as I let out the clutch. It seems to only do it right after I use reverse. I've already tried changing the timing of the gear change (shift to first while still backing up, wait for a complete stop and let the centrifugals slow down then shift, etc...). Nothing seems to make it better or worse and the really annoying part is that it's very inconsistant as to when it does it. Sometimes it does, other times it doesn't (most of the time).

Sorry if this is a repost. I tried the search but didn't get any relevant results. Thanks in advance for your help guys.

---Chris