I have recently started putting premium 95 fuel in to get better fuel economy as i have been getting 150km out of roughly 24L or 3 quarters of a tank of fuel in my 1200 ute. I ran the first tank of 95 through it and it ran fine after putting in half of a tank of premium for the second time it was running as usual but all of a sudden when i start it and it wont start and takes several attempts once it does it bogs down and stalls i managed to get it going and every time i stop in first gear it stalls but will start again first go. it runs fine once you get past 2nd gear but when you are going back down through the gears it backfires and rumbles real bad. It has just recently been serviced it has all electronic ignition high performance coil new high performance leads spark plugs changed 3000km ago extractors and 2 inch exhaust and a weber 32/36 carby which i bought brand new and fitted about 5000km ago it all ran fine up until this. Is it a bad batch of fuel, fuel pump, or just cant run on premium fuel?

Thanks