I have the exact same problem, I noticed today that the carpet is pretty oily. It first happened about a year ago when I let the car sit for a couple weeks in the sun. I assume there is some sort of breather for the transmission that is clogged. Then as the temperatures go up and down, fluid is forced out through the easiest vent. If the breather were working this would never happen, since the top of the speedo cable is much higher than any part of the transmission. Can anyone advise where the transmission vent is?