DIY quadcopter very unstable flight

ArduCopter Copter 3.3
1

I have a quadcopter that I designed and built from scratch. Its quite large and holds a NEX-5N up the front and a 5S 10A battery at the back, so the weight is quite biased in the pitch rather than roll.
Here’s a picture

IMG_20160418_212531-1.jpg1836×1377 842 KB

Im on PH copter 3.3
My hover is at about 55%
When I hover in alt hold or pos hold, it is very steady, but when I try to auto-land, it starts to oscillate back and forth in pitch at an exponential rate, until it finally just loses control.
When I fly into the wind, it seems to be a bit more stable, but when I turn it round, it starts getting wobbly. Im not sure if it’s me or the FC though.

I had a slight crash a few weeks back due to this, and have now re-built, and am still having these problems. Im a bit worried if I try doing an auto-tune that i’ll have problems and crash, so im hoping I can get some advice on how I should manipulate my PIDs to get it a bit more stable before doing auto-tune, or if there is anything else that I might be able to change.

Here are my logs:
Link to logs

Thanks in advance.

2

You have some puffed batteries there, be very careful. Is it rigid with the arms locked out?

http://ardupilot.org/copter/docs/autotune.html

3

Yeah, that’s my spare battery, it’s see better days.
The arms are very rigid once locked in, but when I had that crash is after I had a rather hard landing the rear right arms slightly dislodged without me noticing, then I flew off again and it totally came out and eventually caused the crash. Moral of the story is that i keep constantly checking the arms at every opportunity.