Since your problem is ONLY related to what speed the vehicle is going and not the speed of the engine that does rule out a bunch of typical car related problems.

The only thing that I can think of driving at higher speeds would be vibrations... something is wiggling at that speeds when its not supposed to.

Does the car drive ok at that speed? Or does it just throw a code going above those speeds?

Maybe try when the car is idling to wiggle "GENTLY" on some of they key components and see if you can recreate the problem that high speeds would cause. like the MAF or injector connectors if you can get to them. Possible the ignition components as well.

Since its multiple cylinder misfire Probably not going to be specific cylinder related issue so I would check main components first, then head down to individual things like spark plugs and injectors.

Hope it goes well!!!

-Darren