P1123 on 1996 Hyundai Accent 10th Anniversary Edition

HELP!
It's a 1996 Hyundai Accent 10th Anniversary Edition, it's running slow like my grandmother riding a bike and the scanner detected P1123

Problems:
1. Slow no matter how much I press the throttle, it feels like someone is holding the car from behind
2. sometimes it shifts down then up again without reason and make the car rides a chunk-chunk
3. hard cold-engine starting
4. The pipe's before catalyst O2 sensor's wires was cut! Why? I don't know, just found it "wireless"
5. DTC P1123

What's done:
1. O2 Sensor was replaced by an universal one - I can't find an OE one for this edition here in Southamerica- seems to be working because the exhaust gas emissions were normal.
2. MAF, presure regulator, ECT, catalyst are news
3. Injectors are cleaned and working
4. No leak at vacuum.

The problems remain...

HELP PLEASE...