Clutch pedal woes. It's a very common problem.
The wet boiling point for DOT3 and even DOT4 is way too low. Most people are running either DOT4 Super (aka DOT5), or a synthetic DOT fluid. ::WARNING:: If you choose to run the synthetic stuff, which has a much higher wet boiling point, you CANNOT mix it with any of the previous DOT fluids (DOT3, DOT4, DOT4 Super). It will turn to a gel. Flush the lines and suck the new synthetic stuff thru. It's much more expensive, at like $12 for 8oz. I think. IIRC, the problem is that the lines/fluid gets heated up too quick, then cools quickly, leaving water in the lines, which does not help the action of the slave cylinder. The design of the slave also lets dust in the line, which if not cleaned, will break down the seals of the slave over time.
It's recommended to change the fluid weekly or before "spirited driving." Usually the problem only comes up on fast, high RPM shifts. I can drive my car for weeks shifting regular, or even flooring every gear, but shifting easy at high rpms, and no issues. But if I try to shift fast at high rpms, it's a no go. You'll also notice that the clutch pedal will be sluggish, have a non-existant action, get stuck down, or be very very hard to depress. All symptoms of this problem.
Here's the article. There's another link to the actual article in there.
http://forum.grrrr8.net/showthread.php?t=4791&pagenumber=&4791-Boiled-crappy-factory-clutch-fluid_=