Distortion problem

Any ideas on isolating a distortion problem on piano music? Experience the same problem with two different sources into different inputs. I replaced the interconnects - no change. I have not heard the problem with other music, nor have I noticed it in the past. Problem is in both channels. Actually sounds like something is loose, but it does not happen often. Also does not seem directly related to volume.

I have Pioneer Elite receiver to a Rotel 1095 power amp to Thiel 1.6's. I know piano music can be a challenge, but the problem seems new and happens less than 5% of the time. Thanks for any ideas.