my car got the same symptom but only when i go over water. running in rain doesnt matter, but whenever i drive over a water pond at around 40mph, i could run only less than 20 mph. Hoewever, the car always comes back normal after 1-15 minutes of driving under 20mph. this has happened at least 4 times until now, and i dont have any clue.
it looks similar to your case,
mine is 1990 LS400 with 100k+ on it