Well I'm not the quickest to respond...a month later, and I'm no pro at the Zuk stuff yet...
...This sounds exactly like an upstream o2 sensor. I've blown my front o2 on my Cherokee twice offroading and had similar effects...stutter as if its going to stall then will grab RPM's like a bat out of hell, popping noises from the exhaust, loss of power and speed at 70ish. Moisture gets in the sensor and blows it. Easy and so far only fix I've found: Replace o2 sensor.
Hope this helps if your still trying to figure out the cause.