I would have to think that direct sunlight is definitely going to make it difficult to keep consistent. You said it was stable at 65 until you plugged it in. So it was still getting direct.
sunlight but that did not make it fluctuate?
When I first got mine setup I was incredibly OCD every time it changed +/- 1%. I still check it every day but as long as its no more than 3% off my target then I don't even think twice. And even then I usually just wait a day and then its back to normal. I have accepted that its just going to fluctuate. Plus unless are are spending mega dollars on a nasa level instrument...these hygrometers are not going to be 100% accurate anyway.