Ls376/480 wiring, ecu problem?

[KSN]

I have a prosjekt car thet is nearly 100% ready for the road(1970 firebird). Have had the car totaly apart.
It has a GM LS376/480, 2010 production, GM original computer and wiring harness for this motor, monted with a 700R4 stage 2 modified, and a 2000 stall comverter.



The problem is this: When i start it up and let it go some seconds i put it in gear and the engine dies out. It does not compensate when it get some resistance.
I am pretty shore the setup is correct. Its no warning signals from the computer. Its some better when its warmed up. I can then put it in gear and drive,
but it fell down in RPM before i use the trottle pedal. It also has a tendency to fall down to neary 0 RPM
and nearly die when i get of the trottle quick when driving. When driving in constant speed or acerelation it works fine.

I am living in Norway and its not many that have knowledge about this engines.

Is it any wiring or ECU problem? Think an expert can help me out.