Quadcopter flight speed

I have been wrestling with this problem for several months and I am making zero progress. I am hoping somebody can provide some pointers or tell me that this is simply not possible.

What I am trying to do is create an Ardu SITL simulation of either a quad or heli that can fly at a forward speed of around 60 to 70 m/s. When setting up the velocity parameters on either the quad or the heli, when either frame gets to a speed of around 30 m/s or so, it starts to oscillate between a nose down pitch of around 45 degrees and level flight while trying to maintain speed an altitude… which is expected.

My question is, are there parameters that I need to look at that would reduce this pitch oscillation and allow the SITL to fly at 60 m/s without loosing altitude?

I have been able to get a JSBSim C172 (full size) with ardu sitl to fly at 60 m/s but this brings in fixed wing issues and I really need a vehicle frame that can hover. My initial thought was to use a Bell 407 in JSBSim connected to Ardu, but I can’t find a model that works. It appears the data structure in the Ardu JSBSim interface file does not populate all the servos, so I am not sure the heli control mapping will work anyway.

Any advice on how to proceed?