1999 BMW 323 clutch

1999 BMW 323, stick shift mileage: 85,000. This car is in very good shape. I have no complaint except. My clutch. The car runs great around town and on the highway, however at low speed in first gear or reverse, the clutch freezes up momentarily. For example, if I'm parallel parking, and have the car in reverse, I can feel the clutch lose pressure, and I have to use a great deal of effort to get the car out of gear. When I release the clutch, and push it back to the floor again, I am then usually fine. On rare occasions, I have stalled the car because the car acts just the same as if I slowed to a stop in gear without depressing the clutch pedal. This happens only at low speed, and almost always in reverse. It does not happen every time. It has happened a few times if I am in first gear. The car shifts and runs just fine otherwise. I have checked the clutch/brake fluid, the level is fine, and there seem to be no leaks. I have checked the brake/clutch fluid level, and that seems to be fine. Am I looking at a clutch replacement, master cylinder, or something cheap and easy? I want to know more about this subject before I bring it to the mechanic. Any ideas?