Bugger

I have to source, and fit a new (read secondhand) gearbox to my Xantia. After finding the previous owner had run it on what appeared to be a couple of tablespoonfuls of copper-ease, it has improved somewhat with a flushing out, and some fresh gear oil. But, in 4th and 5th gear, cruising along, it has an annoying tendancy to judder, and lose drive periodically.
I'll be doing it down at the car club at work (big, well-lit hangar, with all the vehicle ramps, axle-stands, engine braces etc you can shake a stick at), and I'm wondering if anyone has any pointers specific to the Xantia.....
Any pointers/help appreciated!