i know it sounds stupid, but because the issue is so intermittant, i would check the knee bolster under the dash. mine has sort of 'grabbed on' to my clutch before, making it feel as though there was no return force in the clutch. i fixed the issue by simply re-adjusting the bolster.
if you don't know what i am referring to, it's the crappy piece of thin fiberboard composite bullshit that BMW seemed compelled to put below the steering column on our cars. it used to piss me off that this part seemed so cheap and cheesy, until i found out that it is meant to break easily in case of front impact crash. i guess it's better that breaks than your knees...
