Hi Frosty,
I'm going to be following this one closely so any details you can provide of what find will be greatly appreciated. I have almost the identical issue except my power fluctuates when accelerating hard in 2nd and 3rd. Over the course of a few seconds it varies from ripping my arms out of their sockets to being fairly tame (A bit like riding a Harley I imagine
). The issue isn't as noticeable in 1st and seems to go away in higher gears too, but the bike seems down on power generally in those gears. I get some change in engine note and some popping on decel but noting serious, my bike is also a standard roadster too though.
I'm going to start simple and start at the start with new plugs and tightening the coil connections then take it from there if the issue remains. There will be a lot of suggestions thrown at you over this one no doubt, but my two cents is start with the basics and go from there. I suspect I've either got a fluctuating quality of spark or the secondaries are doing whatever the hell they want so I think I'll move onto checking the TPS after all that.
Goodluck.