Andy, it sounds to me like you are following all the correct steps. Another possibility is a resin build up on the bit. That could cause your bit to stick and pull at the grain. Aside from a bit that is dull or out of specs your problem may be the wood itself. When you assemble your wood into the drum shape are the growth rings all in the same direction or alternating as if you were glueing up a panel or a table top? I am guessing you are having the bit grab on the growth rings and this is why you are getting tear out. What type of woods are you using, and what percentage of moisture do they contain? It is possible some of your wood has cured longer than the rest and is at a different moisture level. Since there is no way to be sure how long the wood has cured unless you cut it yourself, a moisture meter may be the solution. Your wood may be fine at 12% moisture and sticky at 14%.