The UAV altitude holding mode has floating gas level


I was wondering if anyone has any idea about how to solve the following problem.

We are in the process of tuning our quadrotor UAV. When the GPS is on, everything is fine. Both modes: Position hold and Altitude will work fine.
If you hardware turn off the GPS, the altitude hold mode does not work stably. The copter regularly changes the throttle level. The logs show the same baro level. Altitude changes periodically. Sometimes the copter hangs in a fixed position, sometimes it starts to rise or fall.

Any help would be greatly appreciated.