My '97 GTS has started to do something odd. When I open the driver's side door about halfway, it seems to get locked into position. I can eventually force it back closed, but there is a loud pop when I do that like it's being caught on something and then finally pushing through it. I visually can't see anything wrong happening. If I open the door less than about halfway, there is no problem....but is an annoyance. Any ideas what's going on and how I can resolve this?