Ahh yes, the precious metal discussion.
Precious metals are in their own ranking system, the more rare the metal the more its worth, we harvest more of it every day, which leaves less and less of it in the ground, Obviously why it is a precious metal in the first place.
Gold, has been for thousands of years, a way to measure wealth. they would cover the walls of their homes in gold, gold pillars, golden floors, just about anything that they could make out of gold, and could afford to do so, was done. Hell even the wealthiest of the wealthy would make toys out of gold for their children.
for example an ancient Egyptian toy discovered in in one of the pyramids was a golden airplane.

or, for a modern day reference, a 3ct diamond pacifier that Brad and Angelina purchased for Shiloh.

Why am I going off on what seems to be a completely different path then what you asked about in your first post?
Because in order to understand how we feel about precious metals, we have to understand that it has been brainwashed into our DNA since damn near the beginning of time as we know it.
okay, so now on to your original questions.
Are precious metals in a bubble of their own?
Yes, very much so. although economy does change estimated "worth" of precious metals, But, in recent trends, because so many people seem to think that the world is going to end < yet again > they are buying more and more precious metals, but its not just about that, Also many people are becoming less trusting of the stock market, and are investing their earnings in precious metals. Another interesting fact, is that people are actually requesting that the gold be sent to their homes upon purchase, which is different because historically speaking, when a person bought a brick of gold, they kept it at the bank they purchased it at for safe keeping, which also tells us that people are becoming less and less trusting of the current banking system. Either that or Less wealthy people have started buying more and more gold, and want to hold onto it themselves,, I feel that it is a combination of the two personally.
The thing most people don't understand is that precious metals are only worth what people are willing to pay for them, and if situations become drastic to the point some people think they will become, they need to remember that a brick of gold more than likely wont buy you a sandwich or a glass of milk.
what would personally make me sell any gold I had?
If the current market for gold, had a boom, and the payout became large enough, one would have to assume, that it would eventually go back down below the margin they sold theirs for. Typically precious metals move like stocks, they go up and down frequently. But just like stocks, there are always risks you take when buying and selling. although precious metals "Historically" speaking have had less risk then most other investment types of modern day, making it a much safe < but not foolproof > way to invest your money.
As far as the investment scene goes right now, I would say that investing in certain precious metals might be a smart way to make some growth, but I would like to discuss a few other investment types that people often overlook that I feel in a current market is a smarter way to protect and promote growth of your assets.
These are not listed in any particular order.(1)
Land I feel that in our current state of the world, that investing in real estate is a very wise move. property is at a all time low, many people have lost their jobs and in most cases their homes and land to the banks, often the banks turn around and sell those said properties for a much less value then they are worth. BUT you have to be willing to hold on to the land for a while, it is a buyers market currently, so a quick "flip" while it may land you some profit, wont be as high as if you held on to the land for 10 to 20 years. if you can afford to buy some rental properties, these are also a great way to make some profit while waiting for the land value to increase for sale also. when people lose their homes, they need a place to stay and renting is becoming a more popular option then before due to lower credit scores.
(2)
Small businesses There are so many grants available for small business owners right now it is not funny. Might I suggest a storage facility on that property I mentioned above, Storage facilities are becoming more and more popular under the current times, requires very little to operate, and can net you some fast cash while you are waiting to turn that property into a profitable sale. Pawn shops also are on a come back, but requires more resources, has some risk to it, and generally brings the wrong crowd to your place of business. but hey, anything for a quick profit right?
(3)
Education As Benjamin Franklin once said "An investment in knowledge pays the best interest". Knowledge is power, always has been, always will be, and what better investment to make, then investing in yourself. You don't need to gain knowledge just to get a job, so even if you already have the job of your dreams, go back to school for something of enjoyment that you always wanted to pick up as a hobby. Educate yourself on every day tasks, basic carpentry, auto mechanics, computers, the list of things that could help you not only save money, but perhaps generate more income is endless. There are also tons of grants available for education right now as well, take advantage of them, and invest in yourself and your future.
My only closing statement in this whole topic is that "Investing" in risky avenues, should be well thought out, and planned. if you plan on buying Precious metals, stocks, or any of the above suggestions < with perhaps education excluded from this > Don't invest money you can't afford to lose. if you have 1 dollar left for the week, don't waste it on a scratch off ticket.