Hot day + long drive = no power...

[RG]

After a long drive on Friday from Ballarat to Phillip Island my car decided that it was a powerless pig. I filled up at Mobil on the ring road with their 8000 fuel and all was fine until I gave it a bootfull just before I got to the Island. All it would do was hold gears and make a lot of noise but just had no balls, to the point where it struggled to make it to the top of the rev range. As soon as the weather cooled down it came good but the next day it happened again, this time after only half an hour or so. I drove to Dromana and filled up again with BP ultimate and all was good again, when I opened the fuel cap it seemed to have a LOT of pressure built up. Went well until today when I filled up on Ultimate again on Stud road, not as bad but still very noticeable. WTF is going on, it's obvioulsy a fuel issue but why so intermittent? Are the servos using a blend that is not intended for such hot weather or is it just my car really not liking the ambient temparature? And before anyone says that cars don't make as much power on hot days, I know that but this is way too much power loss to be normal. Any help would be appreciated.