Thanks @Chad-Sweet for your reply. So I did not try the PID method, but instead used a new 3S lipo battery to replicate the previous flight behaviour. The first test was a huge success and the drone flew well even while facing the other sides. But further tests, the power avionics issue has come back again and the tests are now inconsistent. Sometimes it flies well and sometimes its going haywire, but almost getting the avionics power low issue every time.
Could this be the power module issue? Anyway I've ordered power module and my next step is to replace with a new power module and test. But is there anything that you could suggest?