but every time difficulty increases, the price go up
Then we have a bunch of stupid traders on our hands that can't look a few days into the future at a pre-scheduled event.
The price for the next difficulty increase is already baked into the current price, because everyone knows it's coming. This isn't to say that the price might not increase anyway, but it will be for different reasons. Honestly, the amount of bitcoins sold on Mt Gox that are freshly minted is only a tiny fraction. That tiny fraction getting a little smaller shouldn't have a big effect on prices. Price increases are mainly due to popularity increases, not difficulty increases. Demand is the driving factor. There are already 6,500,000 bitcoins out there. The 7,000 mined each day aren't that big a deal.