i had a problem on my old st with no real pull when you wanted to give some with my right foot,i took both st,s(mine and the missus)to the dealer who drove them both back to back,they could not find what the problem was,the missus st was a rocket,eventually i got shut and now i have another st which is potent.
i would take it back to dealer and get the service manager to take your st out with you and see what he thinks.
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on my 2nd st.