Sounds like a backfire. Since you just changed fuel and all, disconnect the battery for about 30 minutes and have the ecu recalibrate. See if that helps at all. when you start it up, let it idle for a minute or two then it'll calibrate while you're riding it for the first little bit and hopefully back to normal. Can't promise it'll fix it, but it's worth a shot and it's free.
06 Stratoliner S - far from stock
08 Lexus ISF
06 Lexus IS350