It sounds a little like you are 'stalling' the control surfaces.. Such as in fwd flight and you decrease power and increase elevator until the nose drops..

You are entering a fast fwd loop with all the elevator.. that creats a lot of drag and turbulance..

Try the same thing with just 3/4-back elevator... I'd bet the loop smooths out...

After that, if you want a faster loop, some people set up a '3D' plane to have flap input with the elevator.. You could mix down flap with up elevator, this will help push the plane through a tight loop...

You can also set up dual rate for fast fwd flight and use something like 70% total control throw.. Just remember to switch back for slow-speed flight

Let us know how additional tests work out..