It sounds like a slow cooling of the wort is the likeliest candidate, especially if it is only happening when you have a high percentage of light malts (pilsner, maris otter) as your base.
Have you tried stirring your wort in the opposite direction of your chiller flow while cooling? I've found that, while tedious, stirring has shaved a good 20 minutes off of my cooling times. The other thing that has worked very well for me is adding salt to my ice bath. I'm able to cool my wort in less than 20 minutes now with my homemade IC.
My tap water is a bit colder than yours, but by trying those two additional methods I imagine you should still be able to reduce your temps to pitching range in 30 minutes or less.