This has come up on another thread
I had exactly this problem on an XS1100 years ago and after a lot of money spent, and banging head against wall, it turned out to be one of the HT leads arcing.
Wrap some electrical tape around the plug caps to just passed where the lead enters it and try it then.
If one or both of the HT lead has broken down, this will insulate it and stop the problems under load.
If it is one of the HT's, replace them both as they're not that expensive.
If that doesn't work, check the condition of your plugs, make sure they're torqued in properly (not too tight), and make sure the plug caps are seated properly onto the spark plugs.
I don't think this is a problem withthe fuel system.
If it were, it would manifest at any given time, and not just at higher speeds.
I reckon it's electrical.