SCOBY

I,ve had my Terzo about 5 wks now, in the whole time i've had it i've been using NUL, on start up from cold it sounded perfect no unusual noises ( PERFECT), at slow speeds it was running lovely and smooth, no jerkyness no hesitation at all also on pulling away it was fine. I had the ECU reset a couple of weeks ago this didn't
seem to do much apart from bring the revs up a bit.

Couple of days ago I thought I would fill the tank up with SUL cos everyone says it makes it run a bit better, it was from SHELL but not the OPTIMAX fuel.

Everything I have described above has gone totally the other way now its starting to run very hesitant, noisy rattly engine from cold, basically im gutted and wish I had stuck to NUL, it sounds and drives like a bag of **** now.

Please tell me what I can do to resolve this problem. Its only done 37k & its got a full subaru history, the only mods are s/s backbox & K&N panel filter.

PLease help me sort it out I would really appreciate it.

Thanks very much

SCOBY