1979 SR650 sticky clutch when hot

Summer is here and the daytime temperatures are going up. Brief history: purchased the bike last year, after a local shop checked it out for me I started riding just after the holidays. I have put over 1000 miles on her in 6 months, almost ready to turn to 10,000 miles on the odometer. I took surface streets to my destination and I think that I hit every single red light. On the way back home the clutch became sticky or grabby in the friction zone in first gear. This was a 20 mile round trip, about 45 minutes one way. Is this something normal, I hope not, clutch cable or clutch itself problem? Otherwise the friction zone is ok for a 45 year old bike. 
I was just riding a new Honda rebel in my certification class, although I mostly rode in 2nd gear, the clutch was nice and smooth. Oh and I passed, now a trip to my local DMV to get an “M” on my driver’s license. 

fd650d

Have you adjusted the cable per the factory service manual procedure?  Greased the release mechanism?  Lubed the cable?