Nissan Repair: Skyline r33 automatic changing gears unnessisarily, vehicle speed sensor, skyline r33

Question
QUESTION: Hi I bought a 1995 skyline r33 RBDET25 about a month ago, sometimes when crusing the auto gearbox is selecting down gears without me adjusting the accelerator pedal and then selecting back up with no real change in speed, I have noticed a few other symptoms which I'm not sure if they are connected or not, first being when the ignition is turned on the spedo always reads 25MPH, second when i fitted the stereo i had to run the ground cable from the stereo to a metal part of the body as i couldn't get 12V across any of the connections in the cars existing harness. third when the stereo/dash clock/aircon etc is illuminated, if i put my lights on all these illuminations dim. I have a feeling that there is an earth fault somewhere. can you please help.
Cheers
John
ANSWER: John,

First, replace the vehicle speed sensor and that should correct the transmission shifting uncommanded problem.  The second is the weird way Nissan wires radios.  I would run power from the fuse panel to power the radio and just use the dash line to feed the light in the radio.  Or, you can get a connector to plug into the factory harness and that will correct the radio problem.  There is a wire that is ground when the power is off and power when the car is running and there is a ground that is switched too.  Interesting wiring and that screws with the system when using an aftermarket head unit.  So, get 12 volts switched and constant from the fuse panel and that should solve that problem.  You are really lucky to have a Skyline...

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QUESTION: Hi there, Since we last communicated I have sorted the spedo problem but i am still getting problems with the gearbox sometimes, it is most noticeable at higher speeds,i think when it is selecting between 3rd and 4th or 4th and 5th gears, do you have any advice.
Thanks
John

Answer
John,

If yours is an automatic and it is hunting for gears there are a couple of things that could be going on.  Furst, was the speedometer which you have corrected.  Second, check for vacuum line leaks.  If the computer senses differences in vacuum to see if you are under load or not to shift.  The other thing could be the computer that runs the valve body.  I would check in this order from the cheapest to the most expensive.