Land Rover Repair: Transmission Related Problem, land rover discovery, o2 sensor

Question
Hi, I own a Land Rover Discovery 2001. I was driving it on the highway and all of sudden I found revolutions going too high. My mechanical changed the O2 sensor and did the complete service of the tranny. It was running very smooth after that. I drove for 6 months in the city and found no problem. One day i took it to highway, after driving a distance of 100kms at speed 110km/hr, I experienced the same problem revolution fluctuating and gear shifting. Finally it went to numb mode third gear. My mechanic is could find any fault with the computer. He thinks it is transmission. But It drives good in the city and short distances on the hiway. I suspect it is some sensor. please help me.
Tks!

Answer
It sounds to me like you have a major transmission problem which is causing the transmission to overheat at higher speeds.  Unfortunately, this would require a rebuild or replacement of your transmision.  The O2 sensor, incidentally, is unrelated to the transmission.