I am experiencing the same problems. I am currently running 2x7970s (Sapphire) with the following settings:

cgminer 3.1.1
13.4 drivers

-I 13 -w 256 -g 2 --shaders 2048 --thread-concurrency 8192 --gpu-engine 1045 --gpu-memclock 1500 --auto-fan --temp-target 75 --temp-overheat 80 --temp-cutoff 85

For approx. 3 minutes it mines LTC just fine, but then it either crashes completely or 1 GPU stops working.
I have no problems mining BTC whatsoever.

EDIT: I changed the gpu clock to 1000 and the mem clock to 1600 and it is running stable with only 50Kh/s loss

EDIT2: got a super-bright white screen (crash) after an hour (or so)....changing clocks to 925/1375