I have an interesting case: My HD6870 is a 1GB card.

Driver 15.12 fails to install; it appears to install, but does not work.
After trying many different drivers I found that 15.20 both works and is recognised by the mining software.

Trying to mine zcash from slushpool,  I constantly get GPU errors saying
GPU #0 returned incorrect data! 

I tried -i 0 , -power and many other settings like -old, nothing helped.
What do you suggest I do? Many thanks!
-t