Showing adapter plugged in, but not charging?

When I received my hand-me-down PowerBook G4 I knew the battery was shot, so I just ran it off the cord. It took me a year or two to get a replacement battery but by that time the adapter seemed to be malfunctioning because of the continuous usage.

But after having replaced both battery and adapter just last summer, my PowerBook is having problems again. Now the computer doesn't always recognize that the adapter is plugged in and I have to twist and turn it for it to pop up as plugged in. However, even then it sometimes doesn't charge unless I hold the adapter in place a certain way and of course it's NEVER in the same place at the same time.

Any ideas on what the problem might be and how to fix it?