can 1 octane make this much difference?

I agree with the mentioned above. Sounds like you need to back off the timing or something a hair to accomidate for a little variation station to station. Out here are summer mix of regular gas (we only get up to 91 normally) seems to get pretty watered down, and mixed with 105* temps...it for some reason pings in my (9.7:1) and my buddys (10:1) 318's. Well instead of a detune we run 91/100 octane mixes and the issue goes away.