I was thinking headangle before you mentioned about reducing the travel on the forks. I wonder if even at 100mm your forks have a longer a-to-c measurement and they're still too long?
Hmmm,maybe I will check.
But the travel reduces down to approx 50mm..so even below 100mm the unstableness remained.
I have changed the rear shock also..but with one of identical dimensions and stroke as the OE one.
Its not unridable by any means, and may just be a characteristic of the bike..should have had a proper ride on it in its original guise..
Stem is a little shorter at 90mm than the OE ( 120m ) but I wouldn't have expected that to cause the issue.
On reflection, perhaps slow steering would be a better way to describe the problem...using twitchy was wrong I think.