my horse has the exact same problem. and the saddle was checked and it fits perfect I also use a raiser pad and sheepskin. I don't really have any advice (still trying to find out the problem myself) but I'll be happy to see what everyone else says
no I haven't but a friend recommended a Sham Wow so as soon as those arrive I'll be giving them a try!!Have you tried a thin kitchen towel under the pad?
Hmm interesting note! I HAVE been told that I tend to ride too much on the cantle and need to tip my pelvis more forward...but why would it only be happening each winter though? I don't ride differently in the summer vs the winter each year? :-|This is often indicative of the rider being too much on the cantle. The weight isn't evenly distributed over the bars.