Could be cr@ppy fuel. De-natured Ethanol has a pretty high octane rating so when mixed as 10% into quality ULP it should be nothing but better. I suspect though that the fuel that gets used sometimes to mix the Ethanol into is rubbish to start with. Plus the signs you see that say "up to 10% Ethanol". Hmmm, when I read that I wonder if sometimes there is any at all, which would mean you are getting a 90-91 Octane fuel if there is only 1% Ethanol as oppposed to 95+ if 10% is present.