Marown,
my mounting is very similar to yours, in fact even though I have extensive experience with Pod/Boom helicopters, I followed instructional videos posted on youtube by the USA DJI Agent (tag name iflydji) in every step.
I also have a strong feeling that our IMU's have a problem. the power is available, the weight is managable. the IMU is supposed to sense the errors and the MC is supposed to send corrective outputs to the ESCs. somehow, in my case at least, the Load is causing the copter to descend when in forward flight or other movements , the MC is NOT sending corrective responses properly.
Marown, if you are installing a new IMU, please kindly share your results. I'd like to know where I should be heading in solving this issue..