I am using a gigabyte 280x and a Q6600 on a oldish gigabyte motherboard with a 650w power supply. Currently I have it working at intensity 11. If I raise the intensity to 12 or 13 in cgminer my computer restarts instantly. Have tried putting the higher values in the config and also tried changing to them while mining was active, both fail and reset the computer instantly. I can't work out why this is happening other than maybe my mobo and cpu are too old or my power supply is too weak?

any ideas? As I said, it's working fine (but slowly) on intensity 11.

tried many configs, here is a typical one I use:
--scrypt -u XX -p YY -o stratum+tcp://URL --gpu-platform 0 -d 0 -w 256 -v 1 -I 11 -g 2 -l 1  --thread-concurrency 8192

Going crazy trying to solve this...