you describe the symptoms well in your post. I've had similar for some 4000 miles. When I clean the clutch oil jet the symptoms disappear only to return gradually (or am I being more careful when warming up - as you say clutch works fine when hot).
How many miles has your bike done? Mines an 05 Adv on 30000+. Orig clutch.
Has anyone taken out the clutch booster and reprofiled the ramps with a file or grinder?