Have you got the Pergeot transverse engine? If so the gear linkages may need a little adjustment. I had this problem with my Talbot, the gears did all manner of strange things, like not wanting to get reverse and 5th gaer etc. I got new linkages and now all is well. One of the 3 links has an adjuster thread, so worth checking out. HTH.....Happy New Year!
Paol.