I have a toilet at my house that once every 1-2 hours will run for maybe 5 or 6 seconds but then stops and acts normally. I have checked and nothing seems to be getting under the flapper. The float is the type that travels up and down the tube (not a ball on an arm). What could be causing this?
Probably a leak at the flapper/flush valve in the bottom of the tank. With age the flapper can get stiff, cracked, dirty, etc and allows water to leak into the bowl.
Semi-universal-fit repair parts are available in most plumbing or home centers. Frequently it's sufficient to replace only the flapper, but sometimes the valve seat has to be replaced too. Flapper change is easy; it takes a couple minutes to make sense of the instructions and one minute to do the work.