Flat tire keeps occurring while bike is stationary. MYSTERY
I had a similar problem on my bike. The manufacturer had made the rims too deep and it was causing the tube to conform to the odd extra "U" shape when pumped up. It took a while to figure out why I was going through so many tubes. 

Check with your manufacturer / retailer. I got mine replaced free of charge.

Perhaps your rims have an abnormality near the valve hole.