Weird. On two rigs (1 and 2) I avg about 1,300 kh/s but on the rig (3) with issues it starts off at about 800 kh/s but drops to about 200 kh/s.
On rig 3, with issues, I have an old AMD tri core but on rig 1 and 2 I have an AMD 1100t and 8350. So maybe the cpu is causing problems. But on rig 1 and 2 I get 90-97% gpu usage but rig 3, with issues, the gpu usage only gets to about 40% but drops to under 10%.
So I don't thinmk it is the nvminer but wanted to compare with ccminer. I was mining core coin which ends real soon so I will try another x15 coin late tonight when I get home from work.
Also possible it's the GPU. Could you try and switch GPUs between machines to see if the problem follows the GPU or the CPU?
Carlo