I have managed to last a whole 24hrs with no freeze for the first time in a week! As well as the SSD change, I did several more changes, suggested by forums. But I can't say which one fixed it.
1 turned off Link State power management under PCI Express section of same advanced power options
2 switched virtual memory from auto to manual, upper limit 8192Mb (recommended is 1.5x RAM size, which would have been 12288 for me)
3 used Driver Talent to check for outdated drivers. It showed WIFI adapter and audio chip needing update. Ignored demand for money and went to Intel and Realtek for latest drivers. Only audio was updated.