The Computer Only Works Properly When the User Leaves the Room

I've noticed a really strange problem with my computer: it seems to work perfectly when I leave the room, but as soon as I sit down and start using it, something goes wrong, such as freezing, lagging, or becoming unresponsive. At first, I thought it was just a coincidence, but it has happened so many times that I’m starting to wonder whether there's a hardware issue, a power problem, or something running in the background. Has anyone experienced anything similar, and how can I troubleshoot it?