Pull the power stage off of the fender well next to the battery. It's the flat thing with two screws. Make sure it is clean in back and clean the contact point on the body. Pull the plug off and make sure both sides are clean. Backfiring is either an ignition timing issue or a mixture issue. Somehow, since it's an '89, I'm more inclined toward ignition. You have referred to your RPM sensor. Are you referring to the crank position sensor on top of the bellhousing? That's notorious for intermittent problems and provides the timing for the spark. I'd look there. I have absolutely had problems with new ones. Got a bad one about a year ago that simply would not let the car start at all. I also managed to crank one down crooked once and that drove me nuts with hard starting and misfiring. One more: Once had a car with such a bad oil leak at the trans cooler lines that it had sprayed enough oil all over the engine compartment to foul the plug for the crank sensor. Make sure yours is clean. Here's one: Remove, clean and re-attach all of the ground points. Look for black wires. Can't recall off hand if there is one on the 89's valve cover. Certainly there are grounds on the intake manifold.

Finally, and this one will annoy you if it turns out to be right: Find a known good AMM and swap it out. If the one you replaced was a rebuilt, they are not always durable.

DS