I'm not an expert, however I would like you to try something so we can exclude one possibility. Please turn in CCC Catalyst AI off. That SHOULD result in using just one of two GPUs. Note that HD5970 consist 2 GPUs with internal Crossfire (SLI equivalent). Crossfire doesn't always work as it should. Please turn off second GPU and check if performance changed (for worse or for better). Post results then.