WMSE, Sundays, 9am - noon. Allow me to direct you to their web site at http://www.wmse.org
for their complete schedule. (Hint: it's the link called "Schedule")
There are a couple other shows that may or may not feature Big Band, but as you mentioned, the station is quite (wonderfully) schizopherenic. If you're unsure, they archive every show of the last six to eight months for your sampling pleasure.