2002 BMW 325 Service engine light goes on

Hi there, kind of a strange question and I probably should hook teh car up to get a reading of any codes to help get an answer here but I was getting a service engine light go on. Because of the codes at that time it signified a vacuem leak which I found two and replaced the boot that goes from the air filter to teh manifold. There was also a hose that needed replacing. Changed the air filter and gas filter and gas cap. This seemed to work for a while but not the light comes on again BUT here is the kicker. I fill the gas tank up. The light stays off until about 3/4 of a tank is left. The service light goes on. At about 1/4 of a tank the light goes off and stays off until I fill up the tank and we go through the whole process again. Any ideas?