Hi Dave...
The extra nitro may have pushed the in-flight RPM to that critical vibration mode and caused the foaming. It is amazing what an extra 200 rpm can do
Dropping the RPM back will most likely help.
In the past, the "easy" field fix for this is to put a larger prop on it, to slow it down just a bit. Try going to a 9 or 9.25 pitch using the 20/20 fuel. That should get you back down in the mid 16.5k rpm range.
Let me know how things work out.
Bob