Wierd problem RPM's revving when shifting

[chrisgblues]

Hi, I have an intermittent problem with my 2007 Yaris, 4-door, manual clutch. Every once in a while when I shift gears and let the clutch out slowly and apply the gas slowly, the clutch seems to grab fine for a second, then all the sudden at the end of letting the clutch out the engine revs upp to like 3000 RPM's. So I have to reapply pressure on the clutch and start shifting again to put it back to normal. It's as if the clutch is disengaging by itself right at the end of the sweep. I'm not sure if the problem is the clutch, but that's the best way I can describe the sound/feel of it. It just started happening this past month and it's happened about 4 or 5 times. What do you think could cause this symptom? Thanks.