I'm in the process of working my way through the steering system on my 1988 Ninety. Like yours it is very vague and constant little corrections are necessary to keep the thing in a straight line. The key here is to remove all the play from each component, as a small amount of play in each track rod end, the drop arm joints, and so on, soon adds up to a lot of play across the whole system.
I don't expect any miracle cures, but I'll be disappointed if my Landy isn't hugely improved once I've been through everything!