IF you are using a manifold with vlaves, might want to check the valve that screws into the manifold, they ocassionally do go bad.
Had one that was about 2 weeks old, that went bad and did the exact same thing as you described, after about 6 hours of troubleshooting, found the valve was bad. Had a replacement sent and no problems since then.
Hope you find out what it is.
No matter how much money you THINK you are saving by brewing beer, you are still just pissing money away