Here is something to try. Often my RD will skip under muddy or icy snow conditions, however a quick boot to the mechanism starts it working again. I finally figured what is happening was the g-pulley ices and clogs up. The g-pulley is the upper pulley on shimano mechs which has a certain about of horizontal float. On clear days this automatically centers the chain on the cassette cogs, but when ice or mud gets caked on the pulley can freeze all the way left or right. Now your chain will skip until you knock the pulley free. What i've yet to try is switching the g-pulley with the lower pulley, it has no play. You can only do this with shifters which aren't indexed or indexed shifters which are perfectly indexed. The advantage now is you can fine-tune the mech with the friction shifter to exactly where the chain aligns with the cassette. The lower pulley will stay true since there is no play and if you've got the b-screw setup tight this system should work better. In theory.

Another thing i would like to swap is to a top-normal rear mech, like shimano saint or xtr. These should perform better in the muck with sticky cables since the springs are stronger.