I have had a similar problem twice in my cabbie. The first time it was due to a failing fuel pump. The car would cruise at speed but just pushing the throttle a bit further would immediately introduce a miss. I had about 200 miles to get home and the “cruise” speed slowly got worse. By the time I got home I could only cruise at about 40mph without inducing the engine miss. When I pulled the pump out it was very, very worn. The second time it happened was not long after I had replaced the pump and I couldn’t believe it was happening again. However this time it occurred after I had had the spare wheel out and I discovered that the spare was compressing a fuel line. Once I repositioned the wheel & line it was perfect and has been for the last two years.
So my guess is you have fuel restriction somewhere. I have also had trouble with another car from an old fuel line collapsing internally that gave troubled only when driven at high speed. Couldn’t see anything from the outside though.
Good luck with it.