I've been waiting for someone more knowledgeable to answer you question. The only thing I can suggest is that the cable has frayed inside the sleeve and is jamming. I presume you have a Super Pratic B twist grip - I wonder if the adjuster could be too far in and interfering with the cable travel. The twist grip on my Tornado has long adjusters.
There's a Yahoo Wards group that is quite active and may be of more help.