Zorin 9 UliDesktop freezing or re-starting at various times

Desktop freezing or re-starting at various times. This may be when the desktop first appears or it could be when using an app or webpage. I am using Zorin 9 Ultimate. I also noticed this in Mint's 17. Prior to this I have had no problems in recent years with Linux. I am running Ubuntu 15.10 also, without any problems. To me, it appears to be a possible problem with the graphics but I am not an expert. I have an intel core 7 CPU 920, 11.9 GBt Memory, Gallium 0.4 on NVQ92
OS 64-bit.
Has anyone else been experiencing these problems and are there any solutions. This is the first time I have used Zorin and apart from this problem, I am impressed with the Distro.
Any help appreciated, Steve

Have you run 'system monitor' to see if any applications are using lots of resources? Any applications maxing out the CPU/Memory?

Sorry I have not replied sooner but I have been unwell. I have tried System Monitor and I can not see any programme consuming an extreme amount of resources. While I was looking at the CPU usage I decided to go back to processes, clicked the tag and noticed a sudden peak in the CPU use and then it froze. I wanted to try and print or copy the results of System Monitor but can not see a way of doing this, as I would have posted them here to get some more advice.
Thank you Swarfendor437 for you suggest and help, much appreciated.
Steve

Hi Steve, sorry to learn you have not been well - hope you stay the right side of healthy!