Fixing possibly damaged USB charging port
I seem to be experiencing this issue.
I think it most likely is a mechanical issue. (The phone seems to react to USB cable movement; what makes it difficult to determine is that sometimes moving the cable doesn't help until rebooting the phone.)
After some investigation, I found that there are suggestions to replace the charging port. (One auction—already over—is here.)
I guess I'm answering my own question already, but in that case, this post might be useful to others—especially since it seems that this is one of the most common issues. Actually, I would like advice on whether my diagnosis is right. And assuming it is, maybe someone who's already done the repair could write a guide on how to do it.
It looks like a straightforward change, but I'm a total noob.