This is maddening and a huge time waster.  I'm working on some parts that at times disappear after being modified.  At other times, the part becomes transparent. The only way to fix it is to "change transparency" twice to toggle it back to appearing solid.  At other times, PART of a solid body becomes transparent while the rest appears solid. 

Maybe there's work being done to correct this.  I don't read these forums as a job, I try to do my cad work as my job.