I've got a root canal coming up on Thursday, which is going to end with a cap.
I also have two other caps, one that's two years old and one about five years old.
Neither of them have popped off. I know it can happen, but the fact that its happened to you twice in a year makes me think maybe your dentist is doing something wrong.
I would probably get a second opinion and start shopping around for a new dentist. Maybe the periodontist you like can recommend a good dentist. Ask him who he goes to!
Trust is like paper. Once it's crumpled it can never be perfect again.