Hey Dal. The mortise lock is the 82-218. This lock should work with handle sets that have 3-15/16" hole spacing. I believe that's the hole spacing on your old handle set; can you go into more detail as to why you think it's the wrong one for this door? In the time being I have listed a couple different handle sets that should work with your door: the 82-030, 82-020, and 82-032, the last of which looks to be what you have. Take a look and see what you think.
It certainly sounds like it could be an issue with either the screws or the bosses they attach to in the handle set. In that case, a new handle set would be best. If you haven't already, remove the existing one just to make sure there isn't damage to the door itself. You also might want to make sure the mortise lock is installed securely. The video below explains how it should install. If you do inspect the mortise lock, just take care not to drop it--it's very common for the 82-218 to fall into the door frame on people during installation or removal!
