HELP!!! 1984 gmc jimmy s15 2.8l help cuts out 50-up

hi i have a s15 jimmy that doesnt run good since i did a full tune up on it the car runs ok in town but when on the highway between 50 and up starts to cut out if you dont have your foot in the gas also i found a vacume line that comes out of the back of the intake and in front of the ditributor thats not hooked up and i cant find where it goes any help would be great