I've run into the same problem, and found David Waugh's FAQ to be a great resource: http://mcl1.ncifcrf.gov/waugh_tech.html They use a 3mM buffer of 10:1 reduced:oxidized glutathione. I've tried that and it cleaves my protein without reducing reducing the disulfide bridges.
I'll second someone else's suggestion to add more TEV. That's worked for me as well, as long as the TEV's relatively fresh and there isn't too much reducing agent introduced along with it. Jason
