I just had this problem with my hydrolic clutch this past weekend. I am very careful to purge all the air from the system periodically and completely flush the fluid twice a year. I noticed the clutch starting to loose its release power while on some technical trails, gradually the clutch started to fail over the next few hours. Each time I stopped and turned the bike off the clutch pressure returned only to dissappear again when the vibration started. Back at home I checked the fluid level and it was sufficiently high, but the entire line was full of "frothed" fluid.
My only guess is that air entered the system when the bike fell on its side and I grabbed the clutch, another possible reason for the brake line problems in this post.