Why do my 2:1 sheets twist all up in a breeze?

Have sailed 3 out of the last 4 regatta days in a pretty sitff breeze.  My 2:1 jib sheets are twisting  badly, such that I cant really work the port sheet after 2 races.  Got so bad today that the sheet would not ease for tacking!  Results in backwinding in a big blow - not good!
I have taken the sheets off and "untwisted" them to make it better, but the problem returns.  Have swivels on the blocks on the pendants, maybe this is not good but it should not make much difference?

Try non-swivel blocks. Swivels are not needed and are allowing the line to twist.
If that doesn't cure ithe twisting, change the type of line used for the sheets.
Is it neutral lay, like a Samson Braid? Or is it a 3 strand line?

I agree. No swivel blocks. Try regular blocks. Also if you leave your sheets attached to the jib, remember to make sure not to flip the jib and cause a twist. It won't keep twisting but will cause a twist at the bridle. Consider trying attaching the line directly to the sail as a test. You should not see twisting, If you still do, It is the line, if not it is the blocks.