I have seen before where 2 seperate vehicles(300GT, and 85 300ZX) have had the free play on the pedal adjusted so light that when I pushed the clutch in with Hercules force, they would both make a scaping noise, and would not go into gear. In a nutshell, the throw out bering is pushing in too much and it sandwiches the the pressure plate fingers into the hub of the disc. Both vehicles were fixed by adding a little free play to the clutch pedal. You may be only experiencing it once in a while due to you not Slamming the pedal all the way down.