What's up with my fiat ducato gearbox/linkage?

I'm having difficulty engaging the gears (all of them) in my 1986, fiat ducato, 2.5l diesel campervan. The linkage broke on the end of the gear-stick, which was an easy fix but, now the gearstick is like the preverbial **** in a bucket! I've sured up the gear linkage and with the engine NOT running, the gears can easily be engaged. When the van's running however, it's extremely difficult to engage - please help!!