Yes, I can see that now that I’ve opened the model.
I think it has to do with softened/smooth lines (although I found some hidden lines too) close to each other and how OpenGL deals with them when displaying the model when one is just close behind another.
I think it’s an interesting looking model, but at several points way too over_complex as to creating geometry. And there are rounding issues at ends of T_pipes (end maybe also at ends of elbows) The segment counts don’t match, leading to too many edges close to (= behind) each other.
Also at at least one point intersection wasn’t right, see:
The elbow seems to (and is) continue inside the valve top. (scene 5)
There are bolt shafts missing in some places (has nothing to do with your display issues though).
I’ll have a closer look later to see if simplifying the rounding ends and dealing with the many diagonal lines in T_pipes etc. does displaying any good. Need more time. My “regular” work needs to be dealt with too