a couple of days ago I made a pole about this gearbox problem. Since then I have been trying lots of things.
But it think i found a solution, at least for myself. what I've done to help this is, braking first then wait about a second or so and then start shifting down. this worked for me in my team career and driver career. i went from 2% per lap to about 1% in 5-6 laps.( 50% race distance). it is not the engine I tried it with the mercedes honda and ferrari engines all with one gearbox update/upgrade and a personal upgrade.
I hope this helped you,
have a good