Nissan Repair: Auto gearbox changing gear unneccessarily, skyline r33, dash clock

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
So John, the word is here in the States that the English drink warm beer because their refrigerators are made by Lucus... Any truth to that? ;) They don't sell that car here so I'll wing it. As for the radio, you can not just use any old hot wire to make a stereo connection. Todays cars talk by 1's and 0's. They are fully computer controlled and have multipul systems on one line. If you splice into it to give juice to the radio, you disrupt the signal and things like not shifting correctly, mis-firing, window only goes up half way, etc... Haywire! For your power wire, you should use the cigar litter or power point wires. Both are real easy to get to and nothing to interfear with. You always must ground to ground. Ground is the metal car frame parts. I think what we call a short, you are calling an earth fault. I think you need to adjust your wiring. The speedo is connected to a speed sensor on the transmission. I wonder if the stereo was sliced into the computer harness and causing this whole problem. The speed sensor sends a signal to the car's computer and tells it when to shift. If the signal is askewed it won't know what gear to be in.