"Hard" codes and dies at idle
 

The car is a 1994 Bonneville SSEi. New computer, new coil packs, new ignition module, recent plugs and wires, new fuel filter, new radiator. Car runs down the road good.
The codesseem to be permanent, i.e., they won't stay cleared. I have EFILive V4 which is a scantool using my laptop and a special cable. Anyway, the first code is P0112, "Intake air temperature circuit, low input." On the computer it shows MAT (manifold air temperature) at -50(f), which is pretty low considering that it's 87 degrees (f) outside. So what sensor sends the intake air temp to the computer? Would a bad sensor here cause it to die at idle?
The second code is P0171,"System too lean." I thought this might be the O2 sensor (in spite of the fact that there were no other O2 sensor-related codes), so I replaced the O2 sensor. It didn't help or clear the code. In any case, this shouldn't cause it to die either. What other possible causes of this code are there?
I can clear the codes, but they pop right back up.
Car dies when idling. This doesn't happenall the timeand usually only with extended idle time (2+ minutes). Immediately after it dies (the key is still on), there is NO power to the accessories (power windows, locks, sunroof, radio) and it WILL crank but WON'T start. After a brief shut-down period, it starts right up and runs fine.
The idle speed (when idling successfully) is in the high 600 rpm range in drive, in the 800s in neutral. Smooth idle until it stumbles and dies. So, good idle speed, it just won't keep it up for a long period.
Car runs great going down the road. Excellent power, i.e., it will spin the tires on hard acceleration from a stop (with traction control off), and accelerates smoothly at low, medium and wide-open throttle, so I doubt it has a fuel problem.I just recently replaced the fuel filter.
All vacuum hoses look good. I haven't done the WD-40 test yet, but good idle and smooth running, when it's not gasping its last breath, seem to indicate there are no vacuum leaks.
Could this be an emissions system issue? I'm stumped.
Keith[/align]