i just tried this,my initial hash rate was in limits where it should be and than after couple of minutes started to drop to almost zero,tried adding scrypt for intensity and concurrency but after that cant even start mining.
was using std. scrypt for my card(7850)
--thread-concurrency 12404 --worksize 256 --intensity 18
If you go to the Process Log in MultiMiner you can see the exact arguments it is using to launch bfgminer. Try launching bfgminer with those same arguments and see if bfgminer alone starts to putter out after a few minutes.
Copying and pasting the arguments here too might help (again from the Process Log).