The problem is the port becomes loose and starts to disconnect from the board. That can happen multiple ways, depending on how the port is strained by it's use and where the "faulty" solder was used. It could in fact affect the middle pins first, which would cause the results you're seeing. Or, you could have a bit of dirt or oxidization on one of the pins in any of the connections (PC, N900, or either end of the cable).

Checking the cable for dirt (or using another cable) would be the first thing I try. If not, checking the ports on the PC and/or N900 would be second. Just be careful when poking the N900 port. Being able to use it to charge is great if it's starting to flake. Good luck!