What would cause to often lose power at around 100kmhr and make rough exploding sound in the muffler?

I can only comment on a similar experience with a 2001 Montana 8 passenger vehicle. While driving on a 4 lane highway and pulling into the inside lane to pass a vehicle while doing 100kph ( about 60 mph ) when stepping on the gas to accelerate for passing, I would notice a shake and what sounded like tiny explosions in the engine. It also may have sounded like the old knocking you used to get before electronic ignition. I found I cured a lot of these problems switching to higher octane gas for about 4 fill-ups then using mid grade gas for the rest of my fill-ups. Even though they state an octane of 87 is adequate, 89, 91 is better, cleaner burning, and better gas mileage. I have 1420,000km on mine and am using 9 liters/100 kilometers which is mostly highway driving.