Windows 7 hangs at "Starting Windows" - but not always

A client has have a Samsung NP-RV511 laptop with Windows 7 Home Premium (64 bit).
After a recent Windows update the machine now hangs at "Starting Windows"...but not always. Very occasionally it boots up fine.

When it hangs there is initially a few minutes of disk activity and then it stops. The Windows logo pulsates and does so indefinitely. The only way out is to to do a hard reset via the power button.

It boots OK into Safe mode.

I have done chkdsk - nothing found
I have done sfc - it says some damaged files can't be corrected.

I have done many systematic configurations of selective start up in msconfig, covering start-up items and services (including microsoft services), to try and identify the culprit, but nothing makes any difference.

I have done system restore back to a number of restore points, but nothing changes.

When it does boot up correctly I have use the Samsung Update Plus tool to check all drivers and all are OK.

I am obtaining Samsung recovery media for this particular model and operating system.

In the meantime, can anyone shed any light on what's causing this issue, specifically the intermittent nature, and/or suggest a fix?

Thanks