strange loss of power P30 454

Check how far the throttle valve opens. It is possible that it is opening beyond vertical. Measure the voltage on the throttle position sensor. Make sure it is still changing throughout the full travel. Voltage will top out at 5 volts. Mixture enrichment happens at 90% throttle. If it is hitting too soon, wide open throttle will not be tuned correctly, because as soon as it hits mixture enrichment, it is no longer in closed loop mode. Don't know if that engine uses MAP, or mass air flow. Speed density (mass air flow) sensors can get dirty and stop working correctly.

Poor fuel quality can cause the problem you describe. Try adding a large bottle of HEET and try refilling it at a place you trust.

I had this problem with my old 454.It was a chaffed fuel line. The bad spot was just above the fuel tank. After years the chassis had 'Settled' down lower onto the framing and was rubbing the line until a very small hole wore thru.
Under normal fuel demand it doesn't exibit a problem. Under prolonged, higher fuel demand, as when pulling a long, gradual grade,it stsrts to suck air into the line and eventually starves engine for fuel. I fought with this problem for YEARS at great expense diagnosing & replacing stuff. I FINALLY replaced ALL the fuel lines. Problem solved.

Total miles is just over 53K and at 51K a tune up was done and smog test was also done.

When was the last time it had a full tune-up?.. How many miles is on it?..

Plugged cat or muffler, perhaps?

EGR valve sticking in one position.

Also if the 02 sensor has more that 60K on it you will notice a performance improvement by replacing it (they get lazy and slow to respond). And for that year they're cheap.

A bad fuel pump or the filter on the fuel pump might be the problem.