check your task manager for any background processes that don't look right. if your hardware check comes back with no new findings, its likely that the issue may be a background task eating up your computer resources. antivirus programs like to do this. as do any streaming game services like steam or origin where it can auto update.
the other possibility, if you aren't PAYING for antivirus, is that you have a virus, as they like to eat your system resources as they are spreading malicious code. while this doesn't seem probable if you have any antivirus program, its still a distinct possibility.
check the hardware. could be a failing component or improperly timed memory, etc. i would venture to guess its more software related though. could be a corrupted driver trying to work itself out, a partially installed driver, startup programs that no longer exist on the system, etc.
if your hardware comes back okay, run a virus scan, and maybe a registry cleaner of some kind.