Actually a loose cap would cause "lack of pressure" tightening it might make the leak worse. Take it to your mechanic and have him pressure test your cooling system. It's easy and quick. He'll connect a pump to radiator opening, pump it up to about 18lbs of pressure then watch the pressure guage - if it drops you have a leak. Make sure your heat switch is in "FULL ON" so he gets the whole system. Any pics of the stains? (trying to diagnose from desk - sucks)