Nissan truck

I have a 1996 Nissan 2 door truck 2.4 engine. I have just done a complete tune up and replaced both O2 sensors. The problem is still here of which it will not hold a steady idle. Once started it idles rough and fluctuates between 400 and 800 RPM. Uses up a lot of gas and smells of runnig rich. When driving slow it bucks like a bucking horse. Once at highway speeds it is fine. The service engine light is on. 196,000 miles. I live in Mexico where we can not seem to find a computer. Nissan from the America are not the same as from Mexico. The parts are not interchangeable. I need help!