I agree with you something is obviously wrong and it is picking up excessive vibrations.
It gives me a good idea where to look now.
I think (not sure, this is to validate) that propellers might be the issue. They are balanced, but at some extend, they can't withstand that much aggressiveness...
These are Phantom 3 propellers that have been used on a F450. They look identical to original F450 propellers, but they could be different in material, etc. Since I can't control this, it is hard to say.
In winter conditions something changes.
Is it the propellers that are getting unbalanced with cold?
Is it the Pixracer that is hard mounted to the frame using only a layer of double sided tape, etc.?
Obviously I have to do my homework...
Thanks a lot for your time and your detailed answer.
Keep you posted.