1989 Buick Century Limited engine hesitation.

Hi,
Buick Century Limited 1989, 3.3L, 300,000 km, original transmission (I believe).
Engine hesitation when on load, i.e.: accelerating, climbing, etc.

After a second of hesitation, it will "POP!" in the intake and resume normally.
Doesn't matter hot or cold, the more I push de pedal, the more likely it will do it.
I manage to avoid it by accelerate slowly and gradually but will still doing it sometimes (but less).
The problem seem to be getting worst every month.
Fuel pressure check OK, Air filter OK, IAC check OK, O2 sensor check OK, TPS check OK, MAF check OK.
All readable sensor data via ALDL are within factory tolerance.
Will not do it on Park or Neutral.
Any clue?
TIA.
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Timing is not correct. Either the timing chain is worn, the gears are worn, or it's an ignition problem. Charles
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