Great start, rubbish middle and ending

What movies have you liked when they started but then in the middle thought "What the hell happened?". In my book, those movies would be something like 'The Harry Hill Movie' or 'The Great Gatsby'.

Saving Private Ryan - an incredible 20 minutes start leading to the dopiest story in movie history

"up"

A recent movie, Under the Skin, had an amazing start became deliriously aimless and strange but in a negative way

Die Another Day has one of the best pre-title sequences in the Bond series but the rest was lousy unfortunately.