Mine is doing exactly the same thing. Starting sometime after midnight, the temp will rise to 90 or so and hang around there until the sun gets high enough to hit the sensor. At that point it falls rapidly to what might be a normal temperature (around 8 am or so). It's always worse when it's humid, not so when it's dry. And now my humidity is low, less than half what it should be.
I wasn't sure how to take it apart without tearing something up. I'm ready to throw it all away and get a Davis.