While this pitching up and down is a real thing called phugoid it is a bit over exaggerated in flight sim. This is, after all, just a video game so you need to learn all of the secret handshakes and stuff to get it to work. Watch the VSI and gently use your elevator control to oppose the climbs and descents until it settles then adjust the trim until you can let go. You have to be patient. I’m trying to fly the DC3 around the world. So far I’ve done San Francisco to Reno, Salt Lake City, Denver, Omaha and currently enroute to Chicago. For this you’ll need to learn the autopilot.