'93 G20 JDM engine install

[Mike Stensland]

Help, I'm a grease monkey in over my head.

I recently installed a JDM SR20DE in a '93 G20 that had a bad engine.  The
car has an AT.  I've swapped the intake and exhaust manifolds from the old
engine as the JDM ones are different.  I'm now in the process of trying to
get it to run properly.  The car starts, immediatley revs up to about 3000
RPMs then the engine speed drops down low and it stalls.  After the initial
start I've needed to hit the accelerator quickly to keep it from stalling.
I cannot get it to hold a particular RPM using the gas peddle as seems to
wander on its own.

I've verified the problem is none of the following:
The compression come at 150psi in each cylinder - seems fine (I couldn't get
my compression tester in as tight as I would like).
The Throttle Position Sensor is within reason (.5 volts up to 3.9).
In self-dignostic all the systems the ECU test check out.
The plugs, plug wires, and distributor cap all worked just fine on my 93
classic.

I haven't been able to set the idle or timing because the car will stall if
I'm not pushing on the gas peddle.  I have played with the idle adjustment
screw with no change in the engine behavior.

Before I start my random testing of various sensors, I was wandering if
anyone else had run into similar problems?  Any educated guesses?