Not sure if it is different because you have a 89 and my 93, but I had this exact same problem. I would drive after a bit and the engine would buck and sound like it would back fire when I put my foot on the accelerator after installing a new o2 sensor (my old one's wires were cut, so basically drove with no sensor for about a year.) Well my problem was that that the harness's connecter to the sensor, which has 3 wires I believe, one of which was chewed/cut. Here's a picture:
I just cleaned the wires from the rubber insulator and sautered them back together. It drove fine after that. I would would look to see if your wires are all in good shape. It's an O2 sensor problem because the only variable you changed is the sensor.