Has anyone come across fuel damper failure?

My truck: 2002, 2.3L, 5 speed, 75,000 miles. Engine has power loss and misses badly when the engine is under stress either up hill or after shifting to 3rd or 4th (low engine rpm). Otherwise engine runs well at 50+ MPH but when I have pass another car it starts missing and thus looses power.

1. Have tested the fuel pump (out of the tank) at 79 psi should be 56-72 psi.
2. Removed fuel tank and cleaned tank and pump (was not dirty)
3. Replaced the fuel filter just because.
4. Replaced air filter (was very dirty)
5. Cleaned the MAF with MAF cleaner.
6. Measured ALL voltages at MAF all well within limits including all RPM ranges.
7. Ran 2 fuel tanks with injector cleaner additive at high speed.

Only thing left is the fuel damper any ideas?

Thanks.