I deleted my rant… long story in short… I upgraded my RAM to 16gb from 8gb and the problem has gone away in 4.13 but remains in 4.12.
This is a strange problem but I can live without 4.12. v4.11 still runs OK as well.
For reference, this is my spec now:
i5 3570K - 16gbRAM - SSD - ASUS GTX970 Strix - Win7