Hey guys here is a post I did in another thread for a guy that was having electrical problems in his NG900 which seemed like an ECU problem. My problem acted like an ECU as well this is what mine turned out to be.....
........ read through this thread and it sounds like something simular that happened to my 92 9k. I know they are not the same car but they are the same manufacturer and these cars have a lot of common designs and components. There are two things that I had to fix in my car that you could try they will not cost you anything but they solved all my problems.
First this is a common 9k problem in the trunk or the boot there is a wiring harness the runs down the left side in the trunk it powers the brake lights the trunk pop and anything else you have that is electrical in the trunk. Over the years opening and closing the trunk causes the wires in this bundle to crack and start making contact with each other and thus shorting out. It caused my complete right side lights to stop functioning as well as starting problems, battery problems, radio problems and many other strange things. So this is easy all you need to do is untape this bundle find the cracked wires and wrap them with electrical tape. Everything started working again!
Second problem this is not so common but sounds exactly like what you are having problems with. It seems like battery problems or ECU or wiring or fuel pump, even random problems, mine kept going into the limp home mode while driving.:x The main wiring bundle is the main artery for everything including the ECU. I don't know the NG900 but in the 9k engine compartment this main wiring harnes that goes through the false bulkhead. It has a ribbed locking ring that holds this connection securely together. On my 9k the little pins that lock this connection had completely worn down to nothing and no matter what I did I could not get this connection to stay tight. So I would tighten it up and the car would work for a while and then the vibration of driving down the road it would come lose again. My solution was easy I took a drill bit that was about the size of these pins and made a hole just to the right of each of the three pins and found a little flat tipped screw that fit the hole and screwed them in just enough so that the tip became the pin. I hooked it together and it locked right down. All my problems went away! I am now getting 27+ MPG and the car is running fantastic. I also checked the wires in this bundle a found one that was cracked I taped it also.