I had a similar "tired blood" problem with my 1993 Tonawanda 454, but mine started when I bought it new in 1993. After numerous trips to the "stealership", I was told repeatedly (1) We never heard of that before; (2) Unable to duplicate customer complaint; and (3) Vehicle is within specs..
After relating these woes to a friend, he suggested that I have it dyno tested. I explained this problem to the dyno techs, and they told me that what I described was not possible (acceleration to 50 mph, then a leveling off of power). I told them to test it anyway. When I returned, they showed me the readout, and apologized, as the readings were exactly what I described.
Turns out the distributor was defective from the factory. It was not all of the time that I wasted with the dealer, GM, etc, or the fact that I had to pay for the testing and repair for Chevy's mistake out of my own pocket, that ticked me off. Rather it was the fact that I did not learn the cause of the problem until 92,000 miles. Old Blue just turned over 160,000, is still going strong, and running better than when it was new.
I strongly suggest a dyno test before embarking on expensive repairs. It was the best money that I ever spent.