I have a 2005 automatic G35 coupe. I have purchased this vehicle last year around this time. A few weeks after purchase I came across a problem I have yet to solve. The problem is found when I shift my A/T g35 into manumatic it works as should for a couple minutes then out of no where down shifts as far down as it could. To avoid damaging the engine as it randomly downshifts I would immediately place it on drive. When I first purchased the car it was not much if a deal to me as it only happened a few times every couple of months. I am more concerned about it now since it happens almost everyday. This enables me from enjoying my manual mode. I am currently thinking in modding the coupe and tuning it. I would be very un motivated in doing so if I cant fix this problem as I hate driving in drive. Makes me wish I had purchased the 6 speed. But as I already put so much time in this car I would really want to know if anyone has gone through this and how did they come about fixing it.

 

Pull the shift cover and make sure it's not something weird like a cut wire.

 

I’be changed the shifter assembly already and my valve body and nothing seems to fix this problem..

 

Check the transmission ground on the front of the engine timing chain cover, it's one of the two bolts in the middle top, each one has a wire landed on it. Clean and reassemble them both.