Exit and Save, then re-entering game - save position is NOT correct.

I have a world where "Save and Exit" seems to be totally messed up.
(it's a large world - 229MB)
The bahaviour is this.

Play the game for X amount of time.
Save and Exit.
Screenshot of the save looks ok.
Return to play the game later
Save position is NOT where left and game progression is in fact at some point in the past (often back at a previous save position - but not always - these save positions do seem to cycle too which is odd).

I have lost over 3 hours of game play with this bug and now it seems pretty worthless carrying on as you cannot be sure if the game has saved or not.
It appears to me that

1. the longer you play without a save the more likely it will not save and you it will save at a random point somewhere between now and last save.
2. The further you go away from the last loading point (I wont call it a save) the more likely it is to just drop back to the same point load point next time you play!

I have exported and re imported the world with no joy.
A 3 month old export I have of the world is unaffected - which implies the world may somehow be corrupted.
But thats 3 months of lost "fun" ... grrrr I don't want to have to go back.

If you want a copy of this world let me know and I can add a link as I said its quite large.