98 Hombre has lag issue

[dads98Hombre]

I have a 98 Isuzu Hombre 4.3 V6. Automatic, VIN#X0W8654326

The problem: when I am going up a hill, at about 2300 rpms, I will see the rpm gage start to fall, and it feels like the motor is not fireing smoothly. If I feather the throttle a bit it will pick back up for a few moments and then it will repeat the process (lag). If I step down on the throttle and the transmission down shifts to a lower gear, the service engine light will come on: (Error Code is random: P0306, P0304, P0301).

Attempted Resolutions:
1. Replaced plugs,wires,distributor cap and rotar. (Dealer Ship)
2. Fuel Injectors cleaned (Dealer Ship)
3. Updated computer software (Dealer Ship)
4. Replaced fuel filter, air cleaner, changed oil.

I still have the same problem. Could this be that I might have a plugged up catalytic converter or a bad fuel pump? The Dealer Ship said that the injectors were fine, but those are the only three things I have not replaced so far, would truly appreciate input.