I've been exchanging e-mails with someone who'd had a dealer change his clutch pack and spring. Since the work was done the clutch had worked fine when the bike was cold but once it was warmed up and he'd ridden a little the clutch would start slipping. Any guesses what it turned out to be? It had nothing to do with the heat. The owner figured it out.
Inside the master cylinder is a tiny hole that allows fluid pressure in the hose to be released when the clutch/brake lever is released and hydraulic fluid to flow back into the reservoir. That hole was at least partly clogged. Result, the first few times he used the clutch all was well but the more he used it the more pressure built up in the hose until the clutch couldn't engage any longer..it was like the lever was being held slightly in.
He used a straight pin to loosen and remove the grunge in the hole and voila! No more slipping clutch.