If your toilet is running every so often and shutting off and then turning back on the first thing I would check is the flapper that lets water into the bowl from the tank when you press down the flush handle.
The flapper is attached to a chain either metal or plastic and sometimes the chain might be kinked a little and won't let the flapper go all the way back down to seat properly causing the water to drain slowly out of the toilet tank into the toilet bowl.
Then the valve comes on to refill the toilet bowl.
If the chain is okay then the flapper may be bad and need replaced. The flappers are cheap and easy to replace.
Shut the water valve off to your toilet and remove the flapper and take it to the store to get a correct exact replacement.
If it's older than 4 years then it's a good idea to replace the flapper anyway so it's worth a try to replace the flapper to see if it fixes the problem.
If that's not the problem then you should check and adjust the toilet fill valve.
The way you adjust the toilet fill valve will depend on what type of fill valve you have and you can search on youtube on how to adjust the certain toilet fill valve.