SCV = suction control valve, it's a small component on the high pressure fuel pump. On slightly older models it was (is) a royal pain, causing the symptoms you described.
The SCV also caused the engine to "hunt" at idle, like it would never sit at stable RPM, it would surge and back off continually. If you have that symptom as well, throw in a bottle of injector cleaner and see if that helps, but it will likely need a new SCV.
But, if that's not it, there could be issue with the turbocharger or the EGR. Usually see some smoke associated with those. Do you have any means of seeing the boost level while you're driving? A bluetooth ELM327 OBD2 dongle and an Android app like "Torque" can do this.