Jumping back and forth to try to maximized gains and avoid losses is called "market timing," and it is a bad thing. Mostly because for market timing to work, you have to accurately guess when the market hits the top, and accurately guess when the market has hit bottom (so you can buy back in). Getting either one of these guesses right is extremely difficult, getting them both right is impossible.

If you're young and have decades for your nest egg to grow... stay mostly in stocks. If you're close to retirement you should be invested in a roughly 50 / 50 split between stocks and bonds.

If you haven't heard the story before, you should meet Bob, the world's worst market timer.