That sounds diabolical! The mechanics who assemble the bikes at Walmart are pretty sloppy. If the tire is defect free and the rim tape is properly placed then thinking outside the box is essential. Where on the tube is the leak coming from. Maybe you have a leaky stem or valve?

It sounds like the rear derailer is out of adjustment. If you have a friend who is good with bikes have them look at the bike and see if some finer tuning is needed. You can also just return the bike for an exchange. Walmart is pretty good about exchanges. You may have a lemon and need a cherry!