Sometimes a bad MAP sensor can do this. Of course backfiring thru the TBI can also ruin a MAP sensor so now you've got the chicken-and-egg thing.

I think if you disconnect the MAP sensor it'll still run, maybe not so great but if it still backfires you could certainly eliminate the MAP as the problem.

If it doesn't backfire with the MAP disconnected the solution is not so clear, but you might try scrounging the local junkyards for a used sensor. Lots of GM TBI vehicles use the same one. And you'll find one on every TBI S-10 2.8.

HTH and Good Luck............ed