engine management light comes on on long journeys?

when i am travelling on a long journey the engine management light comes on. when i reach the speeds of between 50 and 65mph it comes.the light turns off itself when it goes above 65mph or below 50mph.it stays on around 10 to 15 seconds during the 50-65 speeds. i dont understand why this happens if anybody knows of this or has experienced similar problems with their cars could you please help. thanks for reading.