I have had the sl6 trans issue and i have finnaly resolved it you need to look at how the gears are wearing.
After looking at mine the gears were wearing on the non drive side so put it simple it was stripping when 2nd gear kicked in and slapped 1st.
Problem was the one way heats up and starts to grab so when 2nd gear kicked it is was stressing the plastic on first gear.
Resolve new one way and new lay shaft ( or just clean the layshaft with the wire brush on the bench grinder i did this worked )
i now don't oil the one way but i do clean the one way after every run i mean every 5min qual i clean the one way just with a clean rag and a tool to run on the one way internals.
Every 30min final new gears and new pinions every 2 or 3 race meetings to make sure i finish.
This has resolved my problems completely on the stripping gears.
The other problem you are saying with the gearbox jumping between the gears-
The 2 speed clutch material is not the best if you go for a run even 5min qual and then pull apart the gearbox grab the metal centre that the shoes sit on and turn the shoes there is alot of play there.
OH i hear you say tighten the grub screws and yes the play is gone no prob you are correct go for a run first 5-10 laps gear box is fantastic then back to jumping gears again.
Check the slop in the gearbox clutch shoes again the play is back the grub screws come loose because of the material is so soft.
Resolve super glue the grub screws and find a slightly bigger grub screw but you will need to clean up the top of the clutch shoe after you insert the bigger grub screw ( just with a hobby knife works fine)
I hope this helps any thing else just ask ill help the best i can these 2 issues i have been through already so i feel im very intimate with my 733 SL6 gearbox now lol