Toilet Runs Sporadically
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.