tempo is not holding

I'm trying to warm up to MS4 but it's a challenge. The latest is that the playback tempo changes every time I look around. I changed the tempo in the playback window in the upper right by undocking the window and moving the slider to the tempo I want, which is 68. I then carefully set it back in place so as not to move the slider. I also pulled the tempo text, half note = 80, into the chart from the left options and changed it to read 68. But even though the text says 68, the playback tempo changes to something else. It's not consistent, changing from 68 to 120 or 68 to 55 or 68 to 75. How do I lock in the tempo once I've chosen it? I've also struggled with some instruments dropping out randomly from playback. I'm playing back what I've got, and suddenly the saxophones are silent. Next time I play that section back the saxes play. Is there a setting fix for either or both of these?