My Ubuntu was unable to locate sensors-applet, though given it was 2009 those posts were made, it may have been dropped. I have psensor which doen’t give CPU but does show HD and GPU… yes the GPU goes up while rendering though I’m not sure what I can do with this information; it’s expected.
I think you are right that for whatever reason Blender is giving various frames to CPU. Perhaps it’s asking the GPU which refuses because it only just had a few and then reverts? I don’t see any option in Blender to insist that GPU should be used every frame (and wait until available) though when CPU takes 5 min for what GPU does in 10 seconds… it would make a lot of sense.
Would speed up my work immensely to get to the bottom of this.