Man... I have similar on my 4000... There are at any speed random slight speed changes, which cause vibrations, I brought it to Dremel SC and they said I have to replace electric schema and the rotor shaft... half price of new tool.... ok I wanted mine to be fixed instead to I agreed, but the replacement parts did not fix problem :( I and SC had no idea what to do. Now I bought also new brushes, and casing... to exclude possibility something not tight ... again no changes - vibrations there and speed has 5% jumping time t time, I`m so angry I also want to try now replace last part - this big coil around the shaft.....