I would suspect that the shut off valve is going bad. Pop your hood and look for the emergency shut-off switch, which is located on top of the fuel pump. It has a bright red lever that says "STOP" on it. When you shut your car off, this lever should be in the down position, if it is in the up position, the rubber diaphram has either just worn out or you may have oil introduced into the vacuum lines, which eventually has rotted the rubber diaphram.
Here is a simple test you can do, first of all look at you car right now and see if the emergency shut-off swith is in the up or down position. If it is in the up position, there is a vacuum leak. Next turn the car on and then push that shut off switch, the car should shut off instantly. If you would like to test the switch even further, you can pull of the vacuum line and hook a guage up to it and see if the switch holds vacuum, or you can just take some tubing and hook it up to the switch and then put the other end of tubibg in your mouth and suck in. When you suck in (create vacuum) the shut off arm should go down and stay down. If it goes down and then slowly rises up, you have a leaky switch. The part is about $19.00, but it can be tricky to replace. If you need more help, just let me know.