Was a bit worrying as I was towing camper through sand hills ,auto changed gears at 2000rpm instead of holding .Luckily was a quick fix Nissan replaced 2 solenoids' no more information. Maybe someone has a bit more on this problem.
It's a common thing in the 7-speed box which was originally in the 350Z (or maybe the 370Z, can't remember for sure). Unfortunately the TCM program doesn't like holding manual gears in 4LO. STX550 owners are plenty annoyed at that!
If replacing solenoids fixes it, there are a number of members here who are about to make a rush at their dealers ... can you provide any more information?