Your type of display problem is usually caused by something like:
(1) Model is downloaded from manufacturer, has a lot of surfaces, some are not connected to the rest, so solid is not created. This causes problems in rendering. Fixing this might be difficult. You could edit the surfaces and try to fix it. Those jagged blocks on the main body near the middle of the view seem to have problems, too.
(2) Assembly has parts that interfere with each other. This causes the hidden line rendering algorithm to have troubles and often means some edges are not shown correctly. Fixing this usually means changing the parts so they don't interfere. Sometimes changing the assembly orientation in the view will fix the missing line situation, too. This problem can happen with downloaded assemblies from manufacturers, too, if they have poorly defined parts in them.