First, remove the useless "TechTool" according to the developer's instructions. Then run the extended version of the Apple Hardware Test, looking for bad RAM.
A problem that started without any change in configuration, and that you can't reproduce, is going to be almost impossible to track down.
Since you installed "TechTool," you may also have installed other third-party software that could potentially destabilize the system. If you want to check for that possibility, do as follows.