I had the same issue eg low to mid 600's. Same card (the vapor X) then I set --thread-concurrency 8191  eg 1 down from 8192 and up to 700 + it went. Try it it might work for you as well. Finicky stuff this scrypt configuring....Been nice and stable since.

 --lookup-gap 2 --thread-concurrency 8191 -g 2 -I 13 -w 256 --gpu-memclock 1500 --gpu-engine 1050 --expiry 1 --scan-time 1 --queue 0 --no-submit-stale