I would suggest lubricating all the moving parts first. After that you may have a issue with the hinge bushings and pins being worn allowing the door to sag a bit. You can also try adjusting the striker on the body. Is it difficult to pull the handle up with the door open? If not, then its the friction between the door latch and striker. Again, that is caused most likely by the door sagging due to worn hinge parts.
Clyde.........75 Monza 2+2