'68 beetle gearbox - pops out of 4th

Hi all,
My '68 beetle keeps popping out of 4th into neutral, not always under load but seems worse on bad roads. I just thought I'd post this to see if there was something I should check before I take the box out and hand it to someone. It had a quick shift kit installed before I got it, and the there is a fair bit of play in the gear lever (side to side).
Any thoughts on things to look at (external to the box, I wont be cracking it), or who to give it to in Brisbane will be much appreciated.

Thanks,
Dan

I would change your shift rod bushing and rear coupler first

read here http://forums.aussieveedubbers.com/viewtopic.php?tid=68078

check the gearbox mounts as well

Thanks guys, I'm off to the shed......