Sounds as though there may be a trickle of water going in to the pan from the cistern. To check you could turn OFF the water to the cistern, or tie the float arm up. then flsu the water in the cistern so that no more water will trickle in to the loo. Leave it a goodly time, several hours say, and monitor if it still runs for a few mins every hour. Actually running for two/three mins is rather a long time but first i would think you need to eliminate the possibility of the above steady trickle from the cistern.