Game freezing often

I have a repetetive problem with the game freezing up.
I don't think it is confined to any particular screen or part of the game.
At the start of a game it seems to play ok, not really linked to map size or graphics settings.
Then, later on, after say turn 150 on standard pace, it can tend to freeze up.
I can ctrl-alt-del it and re-load, but sure enough after a few turns it will freeze up again.

Playing using - core i5 vPro, Intel HD graphics, 4Gb ram.

All suggestions appreciated.