How can I restore Safari history permanently on my iMac?

A week or so ago my Safari history started playing up. I have it synced across about 6 devices (a mix of Macs and iThings) and it's never been a problem before.

I noticed on my primary iMac it had cleared. I managed to restore via a Time Machine backup by resorting the history.db file.

That seems to work for a random time period (minimum a few hours, has been up to a few days!) before the history randomly clears again. I can restore with the history.db file but it's only a temporary fix.

Anyone know what I can do to restore things "permanently"?

Thanks!

[Re-Titled by Moderator]