Nissan Repair: terrano auto, vacuum line, vacuum valve

Question
1996 Terrano Auto, have a problem getting top gear at times, not always, keeps slipping down a gear when at about 45-50 mph.
Is there a speed sensor that shifts gear, or some other more sinister device, at fault, sometimes it will drive normal.

Answer
Mike,

Check the throttle position cable first.  If there is more than a quarter of inch slack in the line, full throttle (engine off) adjust the slack out of the cable.  Makes for firmer shifts.  Second, there is a vacuum valve that tells the computer you are not stomping on the throttle and that would allow the car to downshift from forth and unlock the converter.  Usually, it is a vacuum line that has rotted or is coming off.