Tab being held down?

My computer has been sensing that ive been holding down tab at random times. This makes alot of things very difficult. Ive asked a few places with no answers, tried googling it and couldnt get anything. Ive swapped keyboards, my mouse, unplugged everythig but my monitor and it still continues. It comes and goes every minute or so making it hard to test whats causing it. I really dont know what else to test.