I think it might pressure plate getting stuck on, i have a very similar problem when my clutch gets a bit of a beating at the track. It will go all stiff and not engage as good as it should, the first time it happened it just completely stuck on so it free reved whether in gear or not with the pedel alll the way out.
Sorry if i havent read the post correctly but hopefully im close to what you were talking about, about the best i can say is get the box out to have a look. But in saying that i havent even been bothered doing that myself as its alot of work.