1994 Accord EX Clutch Problem

I had the master cylinder in my 94 Honda replaced just a few months ago to finally cure a slow fluid leak which was causing my clutch pressure to drop and the clutch engagement point to fall nearly to the floor every few months. Now however, I sometimes have a new problem: after driving the car for a variable amount of time (usually in heavy LA traffic where the clutch gets a lot of action), the clutch pressure will gradually increase to where the pedal becomes somewhat difficult to press in, and the engagement point is nearly all the way out, away from the floor. One time it was so bad that the clutch wouldn't even fully engage, even with my foot completely off the pedal. This doesn't happen all the time, but I'd still like to know what's causing it. Thanks in advance for any advice.