98 Chevy has a hickup between 40 & 50 miles an hour but still acc. Fine any sugestions

...again am starting to sound like a broken record, but it's going to be your EGR valve all clobbered up and not operating correctly, now you can check the vacuum or servo what activates this and see if it's tellin' what to do on time, because you don't want this on acceleration, though those jokers think they can 'meter' the inoccuous gas to mix with combustion, but if it goes wrong, the motor will not perform correctly...this is my guess and I'm stickin' to it. Disassemble and see just how ganked up the egr is, and look at vacuum lines goin' to it...always replace with silicone vacuum lines like one might find at a custom shop, summit racing has them, like ninety cents a foot or something-...so clean egr and stick it back on, you'll be happy with the results~

You wouldnt notice an egr fault at that speed that when it works. May be a misfire? Do you notice it under a load?

Good thought tennisshoes. I like answers that make more sense than not!