My vote is mirror alignment.
First check for any evidence of amateur repair with damage to any screw heads. Check for evidence in any screw heads visible through the lens opening with the mirror moved out of the way. If screw heads are damaged then send the camera back with a note pointing out the damage.
To check just what is going on:
- Tape a page of the newspaper (if anyone remembers what one is...) to the wall and make a "+" in the middle to focus on;
- Put the camera on a tripod and set the height so the center of the lens is at the 'X';
- Move back a few feet to a comfortable shooting distance and at a horizontal 45 degree angle to the wall and focus on, and center, the "+";
- Take pictures with the lens wide open.
The results of this test will give you ammunition for returning the camera for replacement / refund if something is screwy.
If the focus is
consistent (it is equally out of focus in either direction) up and down and side to side the problem is not lens perpendicularity. In any case, if the lens was not perpendicular, the focused on spot would be in focus in the picture. A non-perpendicular lens is sort of the norm if shooting with a view camera and it effects the plane of sharpest focus but not the focus point.
If the sharp->fuzzy transition points are equally distant from the "+" then there isn't (or shouldn't be) a focusing issue. In this case draw a pentagram on the floor, light candles at its five points and dance around the pentagram while waving a rubber chicken overhead to resolve the issue.
If the focus is to one side or the other of the X then the lens->focus screen and lens->film distances are different. If the focus is different top to bottom then lens perpendicularity can be suspect.
As the FM has a fixed focus screen, the screen position isn't likely to be the problem. Though if someone has gone in and mucked with things then bets are off. Is the screen the original pattern for an FM?
That leaves the only other possible issue as the mirror position. Is there any gunk on the back of the mirror? Is there any evidence of work on the inside of the mirror box?
Mirror alignment can shift if something goes haywire and the mirror doesn't return all the way. With the mirror down, does the lip of the mirror move if you press down on it? If so, return the camera with a note.
Cameras are sold for a reason - and one reason is that the camera isn't working quite right. A glaring malfunction is an obvious sale killer but something non-obvious can be a reason for someone selling a camera and walking away from it.