"but every time I had the rear end getting "squirrelly" over the crest? I know how a bike handles and the affect of rider input but can't work out why that's happening?"
Don't forget that the Mountain has 2 crest's-the BSB boys get airborne off the first and land after the second. My ST4S was spinning up the back in the wet going up the mountain as the crest unloaded the rear suspension-now that did make it 'squirrelly'.
John