First off I would try to get the codes read. Now, although I have never heard anyone give the symptoms of a KS (Knock Sensor) problem, I am going to guess by the sounds of it that you might have a KS code in there. Seeing as you have the SC model, the KS code does matter to you, unlike the Non SC models. If you have a KS problem odd's are you're X may be in limp mode.

Until someone who knows more about it chimes in, try reading up on KS issues. and maybe include your current mileage, and any work that has been done recently. ie; timing belt kit replaced by 105k miles etc etc....