ohh yeah sorry man I forgot you had 17" rims now that makes 2
Draza that could be because remember the standard ST162 suspension is rather soft, and your engine is a bit heavier than the standard 3sge, I have the same problem, I am currently getting a custom swaybar made up to the strength of the GT4 one, I'll let you know if this fixes it.
I read an article that it's not really good to keep you factory suspension when you upgrade the size of the rims, It mentioned they lose alot or performance because they are designed to be used with that sized cusioning profile of the tyre.