I had that problem last year on my 3p600 when calibrating it last year, but once I got into the field planting I never had the buildup. Basically just cranked the drive shaft the right number of times, shook the built up seed loose from the tubes and weighed it, and then when planting the vibrations from the planter kept it from plugging.
I was running well over double the rate of cereals that you are.