I have noticed sometimes that the image from my SP4805 projector seems to shift from time to time. My screen is the room wall painted with the RS-MMMaxx mixture, with a frame wrapped in black fleece. May not be the best solution but if I recall I couldn't find black velvet locally or something. It was 6 years ago but who knows. Anyway, I made the frame based on the size of the projected image on the wall.
I have the projector mounted on the opposite wall as kind of a large "shelf" setup, with 4 threaded rods and hardware to allow pitch/roll/yaw adjustment of the projector, as shown in this photo:
The issue is that sometimes the 16:9 image appears to bleed into the bottom of the black frame slightly, or sometimes into the top. Sometimes I'll adjust the pitch slightly to get it back in-frame, but then later it might be out one way or the other again.
I can tell you for sure that nothing is "sagging". I checked the level of the upper shelf the other night and it's still perfectly level after 6 years, to the best I can tell with a 2-foot level.
Could the issue just be the house expanding/contracting with temperature changes throughout the days and seasons? This is an exterior wall that the shelf is mounted on. I suppose an interior wall would be better for support, but room layout makes that pretty much impossible.