I had similar issues on my '08. After a day of ghymkana training. The bike did not get abnormally hot, so I don't think temperature had anything to do with it. But there was quite some riding with slipping clutch.

At the end of the day there was no free play whatsoever anymore in the clutch handle, and the clutch was contsantly slipping. So I did what you did: adjusted free play. It still did not cure the slipping entirely though.

Eventually I took the bike to the dealer's. Turned out one of the clutch plates was entirely worn down whereas the rest was still in good condition. Plates and springs were replaced under warranty. The dealer agreed with me that it was not normal wear on a bike with only 10000km and just that single plate worn.