'91 240 Wagon

Symptoms: On cold starts, the engine stumbles and often dies when placed in gear. Infrequently, it will stumble immediately and die. I also have terrible throttle response whether engine is cold or warm. I'm receiving codes for faulty fuel injector/s and rich/lean fuel/air mixture at idle. I only receive the fuel injector code after the immediate stumbling and dying.

What I've done: Checked all fuel injectors as per instructions in my service manual. All work fine. Checked the oxygen sensor. It, too, works fine. I cleaned the entire air intake from filter to throttle body, AMM, and idle air control. Replaced flame trap and S hose. Replaced ignition coil (it was arcing, I believe this unrelated). Checked all wiring in the ignition system, inspected the distributor and spark plugs (all these have been replaced in the past year and appear new). Two of the plugs were slightly sooty, though only around the periphery, the electrodes appearing fine. I cleaned them. Having a '93 240 Wagon as well, Ive resorted to switching parts to see if I can see if the problem transfers and I can isolate a faulty part. I did this with the idle air control valve to no effect and with the AMM to ill effect. Putting the AMM from the '91 in the '93 it died a block down the street. Walking back, I put the AMM from the '93 in the '91. It ran, but not well and being down to my last vehicle I was too fearful to drive it. Returning the parts to their rightful places restored both Volvo's to their original states.

I'm at a loss. Might I be having a failing fuel injector that functioned fine when I tested it but falters at other times? Specifically when the car stumbles completely on starts? Currently I'm not getting codes for it, only after it stumbles completely and immediately on starting. Could this cause my rich/lean code? (I believe it's rich, judging by the smell of gas in my exhaust). And what about the loss of power? I have to emphasize this. It's so bad that I'm fearful to pull out into traffic.