Clutch not working. Very strange circumstances

A friend (who is a mechanic) and I just replaced the clutch on my 2001 Focus (Zetec engine, MTX 75 manual transmission). We took it for a test drive. After we stopped to adust shifter cables in a parking lot we tested it. It accelerated great in 1st gear in the parking lot. A few minutes later while driving down the street the engine stayed engaged with the wheels while the clutch petal was pushed all the way to the floor. Reached the end of the street and turned around in a driveway. After that the clutch would not engage the flywheel. Pushed car home by hand. A few points:
When you let out the clutch in 1st gear it tries to roll forward and will move about 1/2 inch, so there is a slight engagement.
The 4 CV joints all are working ok - no problem with one side spinning and the differential taking power from other wheel.
The clutch pedal has good resistance when you push it down and springs back up - so the pressure plate springs are being pushed in by the slave cylindar.
The slave cylindar was bled until no bubbles.

We don't understand how the clutch could go from working correctly, to staying engaged with the flywheel with the petal down, to not engaging the flywheel with the petal up in about 10 minutes. Mystified and can't come up with an explaination. Help!