Hi TheRobins,
We are having exactly the same issue, we have looked at this before and gave up.
Using the re imaging of a machine as a solution to temporarily cure the problem.
However I have come across this from Microsoft today after having five more users/machines with issue.
To make matters worse for us we have offline (Sync Centre) starting up on some machines making the whole thing a lot worse.
Take a look at the following and let me know what you think.
https://support.microsoft.com/en-us/kb/2610379
Kind Regards