Have you replaced your brooder light recently? Some of the new "unbreakable" ones have a coating on them that puts off a gas that will kill chicks. It is kind of like not heating teflon around indoor birds. There were several threads on here last year discussing this.My brooder is a rubbermade tub with cut out squares covered over with screening for airflow.
They are still on fleece blanket
Not sure about the temp, but they are not huddling or on the ouskirts...they can go right under the light or get away from it.
I am feeding fermented turkey starter