I'm using the 981 on a Pioneer plasma, the 5016, which is the Tweeter version of the 5070. It has the same basic processing and the same panel with a few less features. It works flawlessly and looks fantastic over HDMI. Are you setting the res on the 981? How long is your HDMI cable and what brand is it? I'm using one I got from Oppo. I also suggest calling Oppo as you may have a defective unit if all else is OK.
One thing you may want to try is to be sure the tv is on before you turn on the 981. For some reason, on occasion, my pic looks funny if I do it the other way around.