1988 Suburban Poor performance sluggish acceleration, backfires under the hood at low speed.

Will run at highway speed though not as well as it has, the real problem is the poor acceleration, requires driver to put foot to the floor and it backfires under the hood, has died at low speed on a couple of occasions, and requires a delicate touch on the accelerator to avoid it. I have changed out the MAP and the O2 sensor. it is not throwing any codes at this time. Cat has just been replaced and the TBI is not gummed up, I inverted the breather cover to maximize air flow to ensure it was not air starved this made no difference. Where do I go now?