+1 on trying basics first.

1) Roughen up post with sandpaper where you clamp?
2) Change the plastic piece inside frame that prevents seatpost being pulled out. They do wear out over time.
3) Have you got the brompton self locking frame clip as this has a longer QR than standard clamp ,unless I missed an upgrade.

2) would be my first port of call, also give the post and frame a really good degreasing and drying.