Photoshop :: Frequent Freezes On CS6 13.0 X64 When First Using?

I'm experiencing frequent freezes on PS (CS6, 13.0 x64) when first using it each day. Rest of the time it seems to run very quick and beautifully. But most days I'm finding in the first five or ten minutes of using it, PS just stops. It then takes perhaps five minutes to do anything and if I have used the history brush or resized or anything while it was still fuctioning ok, then there is a frantic catch-up by PShop while it runs through all those unseen movements in one go.
 
Oddly, it won't allow any selections or painting or anything but it does seem to allow you to still look at pull-down menus (even though they don't respond if selected). So, an unusual type of freeze, not a total lock-out, just rendering it useless. Quitting (when it allows me to) seems to revive it and then I seem to be able to work ok.
 
Am on a new iMac with 3 TB Fusion drive - just wondering (worried) it is something to do with that perhaps? All my other CS apps working ok (I use Indesign constantly).