it seems like you RC reciever is going in and out of some preset failsafe positions (you should have none such ting enabled.)
Every time that happens, it also affect roll/pitch and every other channel - as stick mixing is set.
...The log contains ~ 80 occasions of mode switching
To make it worse, the plane is not very well trimmed, so pitch (and roll) integral builds up to fly properly, and is reset on every mode switch ..... causing some nose down and roll every time...
Fix your RX problem, disable it's failsafes or set properly, do range check, and make plane fly stright and level with o hands in manual, use those servo positions as trim.