If I remember correctly (and It's been a long time since I owned a 240z, It was a 71, but I imagine the latches were the same.) the door latch works like this: The door frame has a striker plate (or catch plate) that has a cirlcular hole (like a C shape) with a notch cut out the bottom. The door has a circluar locking bit like a O shape with 2 lugs on it. When you shut the door the first lug strikes the edge of the striker plate, rotating the O shaped bit and it rotates so the second lug fits into the notch in the strikerplate and locks in this position, thus holding the door shut. Untill you pull the handle to open the door then a spring causes the O shaped bit to rotate back allowing the door to open. What happens is the Lugs wear down the corners of the Striker plate, so that when you shut the door the other bit doesn't rotate enough to shut the door properly. It rotates to the first position enough to stop the door opening, but not enough to hold it firmly shut, which is why the door moves, and why you have to really slam it hard to make it rotate to the second (fully closed) position. I've heard the best way to adjust the striker plate is to loosen the bolts, reposition the striker plate, do up the bolts so they hold it in position,but not too tight, you want the plate to be able to move slightly. Hold the door handle in the open position and shut the door hard without releasing the handle and open the door again and tighten up the bolts. I've never tried this. I just fiddled around with the stiker plate untill i got it right. If adjusting the striker plate doesn't work you may be able to build up the corners again by welding (I'm not welder so I don't know if this would work) some metal to it and file or grinding it back to its original shape. Or easyer (maybe, i don't know how available they are) get some new striker plates. If I knew how to upload picture I'd draw a diagram. A picture's worth a thousand words. Mr C