I have had simular proiblems with a customers car of which turned out to be a clogged injector, I took all the injectors out and sprayed then with carb dleaner and refitted with new `0` rings (seals), have not had a problem since.
Fuel treatment are a must these days as modern fuels are rubbish due to the emmision laws and less additives in the fuel (petrol and diesel).
Note....
You should not touch the fuel system unless you know what you are doing as fuel in fuel system is under high pressure, leaks will cause a fire.
Acemotorman
June 2009