It also sounds like you're not getting full engagement if it's "chattering". Is the clutch original to the car? It's possible that it could just be out of friction plating. I would imagine that the weight on the tires and the transmission where it is puts a strain on the clutch, since all that weight gives you extra traction.
I do believe the clutch is original. That would mean that it has 61,000 miles on it. But the clutch does not slip when you pop it
sounds to me like A) air in clutch line, or B) clutch master / slave starting to go. The slave would be my first guess.
i've never heard of a clutch going to cause a pedal going to the floor without pressure. If the clutch goes, the tension still feels the same (because its caused by the pressure plate as well as the slave/master setup), but you don't feel any grab. I would definitely check the clutch master, clutch line, and clutch slave. On a car that old, it may not be a bad idea to replace the whole shebang.
not meanin to knock ya, but unless there is a company named gutrag, its getrag, german, and awesome
(or at least the one in my car and most other bmw's is...)
and welcome