I have this problem sometimes too. It irritation and discomfort doesnt happen all the time, but when it does, its enough to make me squirm.
I have found that riding with my stp/packer helps, as the back, that lies against me is very smooth.
If I dont wear this, I simply stuff a sock down there. Sounds like a stupid thing, but just a soft thin sock (I actually find using those cheap socks you get free on airplanes, the best!) rolled up, in boxer briefs. This way, its always firm agaisnt me, so no moving about and chaffing!