We (still) have an Ektagraphic projector in our large conference room. I'm probably the only one who uses it, but it still works. There are two quirks with focusing it. First, on some slides that are bowed in the mount, you have to compromise and focus on about 1/3 of the way out from the center. On these you can focus on the center where you can see the grain, or on the edges, but not both.
Second, if you project upward at an angle from the front of the table, you do get what you describe. The top will be in focus, or the bottom, but not both. The thing you have to do is move back, aim as straight as you can toward the screen, not at an angle, and use the zoom to fill the screen as best you can.
Hope this helps.