a bit sluggish

Hello all question for you all its to describe so I will do my best. I have a 98 Silverado with a 350 in it, when I pull out onto the highway or anywhere you need to get your speed up it seems to be sluggish until you get to 2000 to 2500 rpm area, after that on the highway it runs just fine. I've tried using Chevron techtron and this works for awhile 4 to 5 tanks then it just doesn't seem as "snappy" my fuel mileage with all this sucks also as i can only manage to get 210 to 230 miles on a tank of gas driving easy the only thing i can possibly think of is is a fuel injector cleaner prof. done. I don't know anybody have this problem and knows what to do or any suggestions would greatly be appreciated.

Could be time for a tune up or you could have a cat pluging.