Not that it is really related to your current issue, but a while back I actually modified an internal bush in my gearbox, as it was way too sloppy in my opinion. Mind you... that was when the gearbox only had about 4K on it.
I noticed a definite difference in how the change was between third and fourth after making that modification.
To that end... the first thing I would check are the linkages, making sure there is no interference anywhere and that there is no excessive play in the joints.
Could also be a sticky clutch, if this sticking is only happening when driving. To test that... if it gets stuck again, feather the accelerator so that the gearbox is not under load from accelerating or decelerating. When the gearbox is in a zero load state you should be able to shift the the gear lever fairly easily back into neutral.