Replaced ECU still have problems

Ok I have installed another ECU. As soon as I installed it the car started and ran ok. It the corrected itself after about 30 seconds. It was running and starting great. I though that my problem was solved. Just out of curiosity i plugged the Old ECU back in...It ran just as well! Not a complete surprise as the car has run right on occasion for a day or 2. So I plugged the replacement back in. The car ran perfect and continued to crank well. I then went to test Drive it. It drove perfect. Then my friends came over and we were gonna go drive it. While taking off I killed it and the car would not restart. It was doing the exact same thing it has done in the past of not turning over and using a lot of fuel. After we let it sit for a few minuites we went out to check the spark. While one of the 8 spark plugs were disconected and had a screw driver in the wire tresting for spark it started up and ran just fine. Since then it has been running just fine. This ECu has given me 3 codes consistently:

23- Idle Switch
24- Nuetral Switch
31- Load Signal

After researching all of these it seems none should cause the problems I have had except for the Load Signal code. Does anybodyu have any thoughts on this. I have been searching all night and am still not sure.

Also when the Engine Control 10A fuse is removed the car still runs fine, when it does run.

This sounds like it could be bad contacts...have you checked all the wires going to the tps, maf, ecu, and neutral switch?? Some corroded wires could very easily cause what you are having problems with...

Well after the car died with the new computer my dad tightend the wires going to the Positive terminal. The guy who had it before me replaced the Fuseable links with straight wires. I am gonna check with Nissan and see if they can get me the fusible links, or maybe an auto parts store. For the past 4 days the car has started and run just fine. I am still a bit scared of it messing up again. Every time I turn the key it worries me that it wont crank. Thank you for the input. :D

The mechanic that works next door to me worked at a Nissan dealership back in the 80's and early 90's, so he knows the S12's pretty good, and he said that 9 times outta 10 those hard to find driveability issues have to do with a bad ground somewhere. There's so many ground straps on these cars (and any EFI car for that matter), that there's just a lotta places it can go wrong.

Yeah I think it is a ground issue, but the computer did help the car a lot. Now when the car wont start we simply mess with the injector connections and it seems to start up. Thanks for all the help The car is running well curently. Lets see how long it last. I hope it is permanent.

Also we replaced the fusable links coming from the positive terminal. The guy who had the car before hard wired them. :(