When you plug into a wall outlet does it charge uninterruptedly?
Personally I think it's your computer's power supply. USB ports draws power from the +5V rail of your computer's power supply. If the 5V rail isn't strong enough for all USB devices you will drop below 5V DC in your USB port. You could look into a USB Y-cable which takes 2 USB plug-ins to maintain 5 volts. If you have a USB 3 port, those go up to 900mA - I would try charging off that port.
Otherwise your best bet is a
(you need an account to see links) (Also you can look into a USB y-cable but I wouldn't recommend that)