Welcome to the Werkz.
I had a very similar issue with my 2000 740iL (4.4l v8) where it would briefly drop and recover <100 rpm at 60km/h and i knew it was happening, so it was bugging me. What solved it (after many other parts were changed ineffectively), was a transmission fluid and filter change. I doubt if your car uses the same gearbox, but its worth a try. Take your car to a specialist transmission service shop and make sure they use the OEM filter and proper fluid, all up it should be around $200 - $300.
Riding in style