The price of bitcoin will not always go up and will not stabilize at some price neither will it always come down.
One cause for the fluctuations in the price of bitcoin is it's volatility. The volatility of bitcoin, being one of the very nature of bitcoin, cannot be eliminated and will always be there. This is why there will always be pumps and dumps in the price of bitcoin.
I'd advise you to sell your bitcoins as soon as there is a profit for you cause no one can say for sure what will happen to the price of bitcoin in the future as there may be a dump or a pump. Sell in order to be on the safer side and not later regret.