I say do what a couple previous people have said:

1. check your valve cap for debris, and check that the valve stem is tightened up. Yes I read the op but bear with me.

2. Check your tube for holes by filling a bucket with water, putting the inflated tube in, and checking for air bubbles. If you inflate the tube while not inside the wheel you wont be able to get close to the types of pressures that most people ride at. This means a leak may only occur whilst installed.

3. Check the liner in your wheel and make sure it's intact. Check that no spoke-ends are poking out and hitting the tire. Check for junk stuck in the tire itself, basically if there's a sharp part anywhere, it can do damage.

This is what I feel to be a comprehensive list of what could be causing your problem. If you check all this, and it seems okay then you're being given the runaround by somebody. Good luck.