That sounds more like a hydraulic issue than a slipping clutch. Check your clutch fluid level and try flushing and bleeding your clutch system. Don't forget to finish off by cracking the bolt which holds the line to the master open while maintaining pressure on the clutch lever. Make sure to tighten bolt before lever bottoms out. repeat until only air free fluid escapes.
NOTE: use plenty of covering under and around the area of the master to avoid getting fluid on stuff you don't want damaged. I usually wrap a rag around the bolt and wrench and listen for air rather than watch as fluid with air in it makes a distinct sound, something like forcing saliva between the tip of your tongue and the roof of your mouth, were clean fluid escaping makes no sound.