Just gonna indulge in a little thinking aloud here...
Ok, so stutters at 10k rpm in 3rd (for definite, maybe at those revs in other gears but not so noticeable), and also when opening throttle hard at low revs in third. So, electrics are under load in both instances. I can't say anything regarding your multimeter measurements as I don't know what they should be, but, they won't show what is happening when under load. I can't help but thinking that stutters at high revs points to electrical breakdown. Older bikes do tend to have such breakdowns more than newer bikes - just materials degrading over time - mileage not necessarily the most important factor.
Still, easiest and cheapest things first, and if that means air/fuel systems, so be it.