Clutch Does Not 'Return' - what would cause this?

'92 Nissan 240SX 140K mi, 4 cyl, put new Beck Arnley clutch in including disk, bearing and pressure plate about 2 years
ago. Also put new master cyl in about that time.

Couple weeks ago it went to the floor and sat there. I can flick the pedal with my foot and it comes up. Then it works
fine about 20 times and might intermittently 'stick' again. I've also observed the behavior from underneath with someone
operating from inside. Sure enough the slave cylinder activates the clutch and it stays down (indicating there isn't
likely a problem with the hydraulics?). Clutch feels fine and engages sharply - no sponginess or difference in 'feel'.

I've never run into this kind of problem before. Possibilities? Weak spring on clutch plate? Something wrong internally?
'sticky' hydraulics?