(While Driving) Sluggish, Lost Power, Transmission?

I own a Chevrolet Tahoe 1999, 4x4 5.7L engine truck. She is my baby, I have already replaced a few things on her, i.e. radiator, O2 sensor, starter, positive terminal from battery to starter. I bought it from New Hampshire (I live in MA), the title has one owner, bought in 1999. 156,650 miles on it currently. When I drive, I feel something weird from the engine, once I accelerate, it kind of drops in power/speed. Hard to explain, but also when accelerating it just feels sluggish, it just doesnt feel right. I checked the tranny fluid, color checks out, the smell I'm not too sure of, but I was wondering if anyone could possibly give me a few tips or suggestions, I've searched high and low on the web for possibilities but nothing really hits the nail on the head. THANKS!