1996 Mazda Miata Clutch squeak

After driving the car several minutes, operating the clutch pedal begins to produce loud squeaks. Both the downstroke and the upstroke squeak badly. But, the noise itself comes from underneath the car (not interior pedal mechanisms). Otherwise, there are no clutch performance or noise issues. Once the car is warm enough to produce the squeaking, using the clutch pedal will cause the squeak regardless of whether the car is on or off, in motion or stopped. Once cooled down, use of the pedal no longer causes the noise. Last year I had the local Mazda dealership do some general servicing and asked them about it. Their mechanic said he DIDN'T think it was the release bearing, but he couldn't reproduce the problem (I think it was a cool day, and they must not have driven it long enough). Any ideas? How should I address this? The squeaking is loud enough to ruin the driving experience (this is a convertible).