in your gas tank. Octane booster will probably only make the issue worse as I have heard the Z doesnt take octane booster to well. Just stay away from pure ethanol mixtures and look up stuff on the injectors. There is a write up somewhere about taking away power to the injectors when the power is off so that they dont keep pulsing I believe.
Ethanol is going to be unavoidable so you may want to start to save money for a newer pintless injector (yellow top, or 740cc nismo).