Using a light source and a magnifying glass, carefully inspect the solder joints on the rear of the board (the side with small surface mounted components), particularly the 4 pins where the USB ports enter the board. On my unit, a few surface mount resistors and ground connections also had some poor joints.
I used a small chisel tip set to 750 degrees F, but you must be extremely careful not to hold the iron a component for too long at this temperature. If you're not experienced with this kind of work, set it down to 450-500 and be patient. I've done this type of work regularly for several years and wanted to move quickly.
Add a bit of flux to the joints to be reflowed and apply heat. I found that I needed to add some additional solder in a few places.