mysterious freezes

[steveplater]

Once a week or so my iMac (running Mavericks) suddenly freezes. I can move the cursor, but not force quit by any method e.g. right click, Alt-Cmnd-Esc, Ctrl-Q – nor even switch off power. I am forced to unplug the computer, then replug and start up again.
This always happens when in Safari, but not on any particular website: can be Facebook, Twitter, BBC News or other. Security software reports no virus or malware; and operation is completely fine the rest of the time.
Any ideas about cause – and especially how to prevent??

 

[honestone]

What version of Mavericks do you have?

 

[steveplater]

10.9.5
Though thinking of reverting to Snow Leopard, as I can't run my old version FileMaker Pro with later OS.