My 93 940 turbo wagon has suddenly developed a severe stumble under moderate to hard acceleration. I'm not having much luck correcting it.

A little background: a couple of months ago, I got a "check engine" light and pulled the codes. It indicated a problem with the AMM, so I pulled it and cleaned it with brake cleaner. After reinstalling and starting the car, the check engine light immediately came back on. I checked the codes again and found 2-3-1, "Fuel trim (lambda control) too lean or too rich at part load".
The car was running fine and gas mileage was as good as ever, so I just ignored it.

2 weeks ago, my bulb out relay gave up (I had already repaired it a few years ago by resoldering all the connections). In the process of troubleshooting that, I pulled and reseated all the fuses. That didn't correct the bulb failure relay problem so installed a new one. However, after that, I found that my "check engine" light had gone out. The car was still running fine.

Last week, the car started this hesitation under acceleration, almost like an ignition miss. It will buck under moderate to hard acceleration but idles fine and runs fine under light acceleration. There is only a slight dip in the tachometer (no sharp dips which might indicate an ignition problem) and boost remains steady.

I replaced the fuel pressure regulator this morning which didn't help at all. I'd tried pulling the AMM connector and found that the car runs WORSE with it disconnected. I disconnected the knock sensor and could tell no difference in the way the car ran, but my "check engine" light came back on and again shows 2-3-1. I pulled the radio suppression relay and reflowed the solder connections on it (they looked OK) and that made no difference.

Any suggestions as to what to check next would be appreciated. Could a bad oxygen sensor make the car run like this?