Are your props spinning the right way? Are they plugged into the correct channels as per the wiki? Do the hand test! Hold on tight and clear to the underbelly and apply throttle - you'll soon see where the drive isn't coming from.
Is your throttle on the right channel? That sounds like your throttle stick is moving your pitch, maybe... Don't know how experienced with transmitters you are.
ESCs individually calibrated with Tx? Correct orientation selection during set-up? Motors all on the right channel as per wiki? I made all of these mistakes. Dig through the archives and everything has been answered somewhere.
If you have not already, remove the weight for balancing.. unless it is extremely out of balance. That is not the problem.
But with a 'flying man' type frame, it will come into play once loaded up with the camera. How much? You are not there yet.
Can you suspend the aircraft somewhere near the CG by a means that will not be fouled by the props? If so, you can determine if the controls are correct. When I started flying my 3DR based frame (home brew) Aruducotper, Elevator was reversed. Even in stabilzed mode it was evident.
Is your DX8 new? Mine DX6i was used and had a glitchy Airleron pots. I could see it on the 'Monitor' setting. You might check the same thing on yours. It should also be evident in Mission Planner during radio calibration if such a thing exists on your system.
Oh, I have to fly my AC in ACRO (fixed wing) mode on the DX6i NOT Heli. Heli slaves Chan 5 with Throttle (inversely). Not sure about how the DX8 functions in Heli. You only want Throttle changing the ESC drive.
Do your ESCs have a soft start function? They don't need that with a quad usually.
Also try disabling/disconnectng the sonar input.. simplify the installation as much a possible.
I used a servo tester to determine if the motor rotations were correct on my quad.
Another idea is a short leash type of tether. Twine about 6 -8 inches in length connected one to each arm. The other end is secured to a large flat board or heavy items that the quad cannot possibly lift. This way you can get up off the ground but control how much it can 'get away'.
Best make sure you are using an aeroplane model on the DX8 and not a helo model. Are you also sure that the assigned motors are on the correct pins?
You can check the control outputs from the dx8 on both the monitor on the rx8 and on the AP Planner. Take your props off when testing this bit.
As a matter of interest, if you turn the whole quad 90 degrees and then try, does anything different happen? If so, check you're really in stable mode with the simple box unchecked.
I don't think the PIDs are at issue (though I have been wrong before ;) ).
Just for the sake of complete information, how is your APM mounted in the frame/body?
There is a 'forward' orientation between 'X' and '+' styles. It is a long shot but I'm just eliminating variables.
For you 2.5 board THIS. The front of the aircraft is to the right.
Your inputs as listed above sound correct. How about confirming the outputs? Is the below the way they are configured? I am assuming that from the HT site I am looking at the top of the chassis.
So the overshoot is either the front lifting or the back lifting? After throttle down it alternates? Wow.
Are you using the adapter board that James Y was showing on his HT-FPV build?