Years ago I used to have a 600 and the symptoms were very similar to what you've described. Upto 6000 revs it seemed perfectly okay, but over that it just wasn't right. I tried allsorts and nothing seemed to fix it. In the end after months of messing with it I decided to sell it. My mate who owns a bike shop sold it for me but before he did it was sat around in his showroom for a while. A customer came and took a test ride on it and whilst this happened the fuel pump failed. My theory is, the fuel pump was on its way out, with a small amount of fuel needed, the bike ran fine, but when it needed a large amount of fuel (i.e. higher gears) then it just couldn't cope. Just incase your not aware, 6000 revs in top gear uses a lot more fuel than 6000 revs in first gear.