'Not Enough' Caster can prouce this intermittent violent shakes at a specifc road speed, usually triggered by hitting a bump or articulating the suspension during cornering. If this is the problem, slowing below the 'trigger speed' should let the shakes stop, & you can often speed up & drive on thru that 'trigger speed' again without any issues - unless yo hit a bump or articulate th suspension during cornering while at that critical speed once more!
With the 'fairly rudimentary' suspension design & the minimal adjustment features on our Spyders, plus the fact that as th suspension compresses & extends from its OE set static ride height during normal use, the only time the suspension & wheel alignment is at 'Manufacturers spec' is when the Spyder sitting EXACTLY at its originally designed static ride height - if you've adjusted the shocks or springs, wound the suspension up or down, increased or lightened the load on the front suspension, or pretty much done anything that means its now not EXACTLY where it was set during construction, then there's a good chance that your steering geometry, front suspension, &/or wheel alignment specs are now no longer set how they were meant to be by the designer.... and that can include reducing the amount of caster (or the tendency of the wheels to return to straight ahead once you release steering effort or once they are no longer pointed anywhere but straight ahead for any reason, such as hitting a bump or turning a corner) and that reduced caster can mean your Spyder gets the shakes intermittently when those 'insufficient caster' conditions are met!! :shocked:
If you have this 'lack of sufficient caster' issue, you might need to find a suspension & wheel alignment guru to fix it, or maybe returning everything to EXACTLY how it was set during construction can resolve the issue, but I expect we'll see more & more of this sort of problem arising as our Spyders age & their springs & shocks start sagging over time & therefore move away from the designed OE settings as they do so!! :sour: