Well that's ruled out the cheap option! I suspect that you two
unrelated problems, though it's pretty difficult to diagnose
without being there......

The next cheapest option is that the detent springs/balls
on the selector shaft have got gummed up, or perhaps the
bias springs are broken/a bit tired - it could be that
something has moved or come loose. Also check that the
remote housing is not cracked (the ally casting that
goes from the botom of th stick to the main box) - they
have been known to split where the selector shaft goes
through. You can get to all this stuff from the top - but
just be careful not to drop anything!
There's not really a whole lot of adjustment available, so
you don't need to worry about that too much.
I'd hazard a guess that it is going to turn out that the
layshaft is moving about for some reason (does it "chatter"
in neutral and foot off the clutch?), and that the
2nd/3rd synchro rings are worn (or the in/out spring(s) have
come adrift), but checking all this stuff first is a very good idea!

That comment about the screws locked with nuts - do you
have any more info on that?

Richard