I only get it when I put her in reverse after driving for a while, long enough for my automatic transmission to heat up enough to finally go into overdrive (it's not the relay or the solenoid, I have replaced them both). Anyways, I will come to a stop, say to park, and if I reverse into a spot, I have to give it reckless amounts of throttle and throw it into neutral just to keep it from stalling, and roll into the spot. When she does stall, I get 2-3-1. I have never gotten it at a stop sign, nor in drive, ever. Only in reverse. I have hesitation occasionally, but never when the car is cold. I do have rough idle at cold start, but it rights itself after 20 seconds (it sounds high, but there is no telling because my tach hasn't come in the mail yet) and the idle takes the car away in steady acceleration when I put it in drive from a cold start.
The MAF was replaced not long ago because it was burnt out by a faulty fully open airbox thermostat which has been removed completely.
The intake manifold has also been replaced recently.
The flame trap is new, replaced while replacing a loose vacuum hose.
The only other crack in the any hose I have found is the outer rubber around the fuel line going to the fuel rail from the back of the engine bay (it's a bad crack, all the way around, enough to see the harder line hidden inside, which is intact).

I have not tested anything else as I am consumed by a broken exhaust stud that occurred yesterday while changing the gasket (Haines manual lists the torque as 23 ft-lbs, which is too much, hence the broken stud. I have read on another forum that the volvo dealership manual lists 18 ft-lbs.)

In short, I figured my case might shake up some of the minds at work trying to find a common link in all of these 2-3-1 cases as they seem quite common. Only in reverse, after driving for a while. Why reverse?