bubbles in the clutch line, need of new clutch cylinder, fluid got hot. I say most likely need of new clutch cylinder, same would happen to me after driving in traffic. After i put a new clutch cylinder in and bled them it was all good.
yeah that was happening to me fairly often right after I had a new clutch kit put in...I didn't know what the hell was going on... Anyhoo, after a day or 2 it stopped happening altogether. Maybe it just needed time to settle in or something, I dunno. Did I break it?