This is just my intuition but the IMC servo seems not so optimized. I can increase the servo gain by 6~10dB easily. And I couldn't see that the PC drive went mad (red) as I increase the gain (=UGF).
The IMC needs careful OLTF measurements as well as the high freq spectrum observation.
It seems that I have worked on the IMC servo tuning in 2014 July/Aug. Checking these elogs would be helpful.