Im doing test with just one 6990 and nothing more and is all screwed... without activating the CrossfireX mode in Catalyst Guiminer only shows one device and is just one core because 330 mhash/s

If I active the CrossfireX mode in Catalyst I see 2 devices in Guiminer (both cores) but after few seconds mining one device shows "Verification failed, check hardware!" erros like crazy!

any help please?!