G'day Tim,
Sorry for the late reply, been up in Forster for a week holiday. Got back yesterday and the forums have been upgraded and I can't post under my old user name PM box is also totally full so can't receive PM's !!
Yep, I'm 100% sure that it's cause of the fuel. Going from 98RON down to 91RON on a car that has lots of timing pumped into it will definatly cause problems.
Like you said, leave the Unichip out until you get a re-tune. Re-tune should only cost you $200 or so, just remember to get them to pull the front drive shaft out if they only have a 2 wheel dyno.
J