i had a similar problem with my S1 stagea[RB25det]
fixed it by changing the plugs
my mech also told me to to check/clean the tips of the coilpacks[where they connect to sparkplugs]
[this is for for factory coilpacks-im not sure if splitfires r different?]
unplug them-remove blk rubber covers/boots & check
also id suggest cleaning out the AAC valve-like the other guy said-with carby cleaner
mine ran a bit smoother afterwards & its easy to do
theres a tut with pics in the stagea DIY section