Besides the risks that @JoeTaxpayer mentioned (which are all valid), you also run the risk that you've experienced a regime change. What I mean by this is that you could be seeing the new normal.
As an example of this happening, consider this chart for gold:
What Happened between 1968 and 1983?
A couple of things happened, but the biggest regime change was the Nixon Shock. Prior to 1971, any foriegn government could exchange US Dollars for gold at $35 an ounce. During this time you wouldn't see gold diverge away from $35/oz because there were goernments that could arbitrage it back. After 1971, the price of gold became free floating, and shorting gold at $36/oz would have ended terribly.
What does this mean for Gold/Platinum
It means you need to make sure (i.e. convince yourself) that things haven't changed. Is anything affecting the supply/demand of either? Will that change back?