I have a sanitary system that I created and is all on the same workset. The problem I have and have never seen before is that when going from Fine Detail to Medium detail some but not all pipes disappear between fittings. When I go back to fine they re-appear. Then for no reason, out of the blue some of the pipe that disappear in Medium detail will reappear in medium but others that were visable will disappear. Also they don't plot. Very strange.
I have done everything you suggest. Then when I updated the video driver to the latest 306.97 (I have a NVIDIA GeForce GT220 which came with the Dell that I have) and after redrawing the pipes that were disappearing everything looks OK.........right now.