It sounds as if the striker has slid out of adjustment and is now too far in.
Normally, to be on the safe side, I would use a #1 phillips (slightly larger than a normal size) to loosen the 2 screws that hold the striker in place.
Since it seems as though the latch catches when slammed shut, try bringing the stricker out a little and see what that does.
You will probably have some trial and error, loosening, making adjustments, tightening, etc... but in the end you'll get it just right.
Also, check to see if the hinges are still true and don't have any play in them by picking up on the door when it's open.
Hope this helps/makes sense.