I was hoping you (and a couple of other whistlers) would have some ideas about what the problem is.
I hadn't considered the temperament issue. I'll say this, though, MY temperament is getting disharmonized!
My husband and playing partner, Tim (aka Reynaud), is now playing concertina, and I'm finding that he must have been doing a lot of compensation on the fiddle that he can't do on the concertina. Consequently, my whistle-playing sounds even worse than usual. I'm trying to switch my D whistle tunes to other instruments to get around the discord. Maybe D whistles and concertinas just don't mix well? (the other whistles seem fine...)