I can move the cube off the engine and put it on the firewall or on the engine mount frame.
What's odd, is that this is the same location as my last RV-7 (same engine, etc.) and I never encountered this issue in 500 hours of flying. Started at 68K and left it there for the duration. Fuel totalized to within .1 gallon per fill up.
Must be a vibration thing - just the right frequency/amplitude, etc.