There was a recent thread on this. I think because the phosphates are being phased out, there is more build up now with some of the newer detergents. I haven't had this problem, but was looking up something appliance-related on the gardenweb site, and there was a discussion there. They suggestion was the Lemi shine/citric acid products used once a month or as needed. If that's working in some capacity when you use it, it sounds like maybe it is buildup?
I think in the thread on the BBB cascade was a brand that was causing people some problems? I use Ecover tabs or sometimes Finish tabs (electrosol) and they've been fine for us so far.
Maybe try something other than Cascade?
Mama to DS-2004
DD-2006
and a new addition-ds born march 2010