Yeah, in your case you are not letting the wort temp stabilize. It took me a while to finally learn to be patient enough to take a proper reading. I have been taking my reading with a large spoon and blowing on that to cool it down quickly, then dropping it on the glass, blowing again and then closing the lid and blowing yet again then wait a few seconds. Problem solved.
That said, I have given up trying to get an FG reading from it. The calculation never seems to work. I'm assuming you have to have a crystal clear sample to get a correct reading.