have you tried to just turn the lath by hand sounds like your lath is spinning the wood faster than the router can cut. I have plans for a router lath that is hand turned with a crank and the results look real smooth maby turn your router up a little and turn the lath by hand at a pace to where you wont burn the wood but just enough to get a clean cut. I may be all wrong i have never used a router on a regular lath befor but at this point anything is worth a try good luck