I have a 2002 ford zx2 and I have to keep it above 1,000 rpm or it will start to cut out, why?

I have done everything possible. I had a full tune up. I have checnged my mass air flow sensor, I have changed my fuel sensor, I know I have a vacuum leak somewhere but don't know where. I think it has to be behind the fuel injector but I would like some input be for I go lookign around.I have a 2002 ford zx2 and I have to keep it above 1,000 rpm or it will start to cut out, why?Yes, a vacuum leak will cause a vehicle to want to stall. Check the intake manifold for leaks and cracks and along the intake manifold gasket.. Sometimes an automotive stethscope will help or a long length of a small diameter rubber hose can be used to locate the leak.