Originally Posted by avs

you didn't mention where the car was twitchy, was it too loose on power and/or off power? also how much steering travel do you have, does it turn in 1meter or closer to 2meters?

i have the s120l OP tubes not the s120lt, but i looked at the 'default' setup on the serpent website and it has some settings that you might want to adjust. (i put my preference in parenthesis)

upper arm insert outside. ( inside for the longer upper arm)
0 toe ( a little toe out like 1 degree total)
track width 198, (a typo, i presume you are in the 171 to 172 range?)
pod droop 1.5mm ( 0.5mm droop with a little sag)
there is not tube damping specified ( 10k to 15k is a good start)
0 caster blocks (is this what you really have? and not the 9degree blocks)
caster shims centered (this may depend on which caster block you have)
no mention of front droop ( near 0)
no mention of side spring preload( near 0)

you might try these changes one at a time: (so you can tell us what mattered)
0) set front droop and side spring preload to 0
1) set steering to 2meter circle and reduce to get around the tightest corner
2) reduce rear droop to 0.5mm
3) longer upper arm
4) more tube damping if car rolling too much in chicanes, less if the rear axle is lifting
5) more caster if it turns in too hard

none of this matters if there is binding in the suspension or some other mechanical problem.

i also noticed, you stated tire size at 42 is that front and rear? you might try a 2mm stagger, leave the front at 42 and add a 44 to 45 rear. does DP mean double purple? you didn't mention if this was a stock motor or mod.