It's prolly just something loose somewhere. I had this happen before on a car I worked on, whenever you shifted hard into any gear it would start running like shit, or whenever you hit 55. I'm thinking 80mph is that harmonic sweet spot that something keeps coming loose possibly. Check all of your connections for your fuel injectors, grounds, and ignition. BE VERY CAREFUL but you can try jacking it up and taking it to 80 (slowly) and checking for anything under the hood. Worth a shot.