How are you fitting inner tubes exactly? They can get pinched if not done right although they usually go bang under first inflation. Not always though.
Partially inflated, tucked into tyre. I'd agree and on my last ride the tyre was inflated and left overnight, it didn't lose any tyre pressure. Went out onto ride and it lasted no more then a couple of miles.
Either way, I was in the market for new wheels but I got a good deal on a new Giant Road Bike so I've opted to go down that route. I have honestly lost all faith in the Cube, and was only to be a starter bike, just getting something newer sooner than anticipated.