There's a dozen things it "could" be.
Verify fuel pressure and a decent spray pattern from the two injectors.
Connect a scan tool, verify ALL the sensors, and ALL the computer outputs including but not limited to electronic spark advance, EGR system operation, EVAP system, etc.
Check all the normal "tune-up" items. Distributor cap, rotor, plug wires, spark plugs. Test the spark power using a spark-tester calibrated for HEI. Check the PCV system. Look at the magnet on the distributor shaft--they're often cracked on TBI distributors.
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And I'd want to know that all the valves open and close like they're supposed to, and when they're closed, they actually seal.
A leaking intake valve, or an exhaust valve that doesn't open can each cause intake-manifold backfiring.