Sand Wrench

I think I am having the same problem.. Mainly in 2nd gear. It happens only under a load. We ride in the sand, and the effect of the slipping is very similiar to what sand in the chain would be. I call it That Chunking Thing. We have been into the case twice now, this will be the THIRD time. Can't find anything wrong, fixed what we thought was broke and it is STILL doing the same thing. All forks have been replaced, the drum has been replaced and the gears are race cut. I am NOT looking forward to opening the case again!!!!!!