Chrisappear

Hello, first post here. I was driving my newly bought 85 12a automatic rx7 home and on the highway I gave it gas to see how it faired at high speeds and in overdrive the engine would seem like it was dying around 85 mph. All of the gauges were reading acceptable. I am not too knowledgable with cars so I have no clue why this would happen. Any ideas? I tried to search but I couldn't get any worthwhile results (possibly due to a lack of the right wording)