Yepp I had exactly the same problem – TFV was for no reason slowing down to 180 kmh and then slowly get back to normal.
In my case, I had 4 tracks lying together, 2 for local traffic and 2 for high speeds. And the slowing occured in 1 direction only! I’ve tried to rebuild the track, no change. Then I’ve completely rebuild the whole big section (1-2 km) of the high speed line, both tracks, and also separated them from the local lines tracks (about 1 track width distance), and it worked!
I also had the same situation later in another savegame, and the problem also resolved by rebuilding a 1-2 km section of BOTH tracks.
Hope this helps!