Hello,
Thank you for reaching out to us about this. System freezing can usually be attributed to hardware related issues. Most notably when system fans fail to cool certain hardware components such as the CPU. That being said, it could also be a result of software bugs or driver issues.
If PC-Doctor is not detecting an issue, this may be due to the issue not presenting itself while PC-Doctor is running scans. Can you let me know which PC-Doctor product you are using? Toolbox or Service Center?
To determine if this is hardware related or software related, it is advised to boot Windows into Safe Mode for a while and see if the system freezes up. Safe Mode will only enable the bare minimum required features and services to run. This will help to determine if there is a start up application or service that is causing the freezing. If freezing occurs after some time in Safe Mode, it is likely a hardware related issue.