All the pictue is going to show is a pillar with no light there :P!
I was running into this at the end of working on my compo map. I'm not exactly sure what was causing it, or on your map, but here are some possibliities:
-Weird architectural shapes seem more prone. I had some rotated weird-angle wedges--they were also only 1-unit thick at the biggest part of the wedge--that were disappearing/reappearing on me.
I made them bigger , and made sure the verticies were on-grid, and then they were ok.
-If you have too many visible entites in that area, they'll disappear too. I found changing some tings "to world" made them stop disappearing.
-If you have ANY other errors on your map--hit Alt-P to toggle "Check For Problems", and check your compile log for errors--all this is worse, and your map will behave even more abnormally, not to mention add more time to your compile