"I've had great results in various coax set ups using the same diameter but different pitch props, eg, 3.8 & 4.5 or 4.7.

It probably won't be as noticeable on an octocopter but a coax hex with different diameter would probably not fly very well due…"

"When I bench test my octocopter I have a similar issue.

But with props on testing while under the restraint of my hand It's fine.

I think its just the apm trying to work out what is happening and slowing a motor down to try get some sensor input"

"I think I found the problem.

New Zealand has a rather magnetic high declination at 24.5deg.

I had left the apm to auto configure the declination using gps... this failed though and set my declination to 0.405!

I think perhaps the copter failing…"

"My advise would be build a cheap quad with the most basic kk flight controller. Get Very good at flying first before you sink any time or money into a large AP platform.

Many people assume flying is easy, based on ardrone or similar experiences.…"

"I'm currently working on a modified HK HAL quad converted to X8 duty.

For better or worse who knows, so far it seems to be okay.  
Posting semi regular updates and photos on my Twitter acct. https://twitter.com/Multicopter_NZ  
It would be flying…"