okay, so the problem i'm facing is a little different, but similar in a way.. let me explain :
on two different instances, my car was stuck in traffic jam, A/C was switched on, and after about half an hour or more of stop and go pressing the clutch again and again for gear shifting..i noticed the clutch got harder to press and the gears were not engaging at a moment... i switched off the A/C and after a while car was functioning as back to normal...
so, i discussed it with two mechanics.. they both concluded there's a problem with clutch slave cylinder when it gets hot due to engine heat (which increases under A/C load), it contracts and constricts movement of clutch that results in getting clutch harder and gears did not engage at that point...
however, i've not gotten that replaced yet due to time constraints
so, if you are facing similar sort of issues, this could be the problem for you as well.