Taurus,
I hear you.
My own mother-in-law lexical problem is even sillier. She's a lovely lady, but she uses hardcore Kyushu dialect, and worse yet, she's not really consciously aware of where exactly the dividing line is between her dialect and NHK Japanese, so when she speaks to me, she just speaks Kyushu dialect VERY SLOWLY. Of course, I have no idea what the hell she's talking about, so then she comes back, using Kyushu dialect EVEN SLOWER!
She's convinced that this business about foreigners understanding better if you speak slowly is just a lie...either that, or that I'm very very stupid!
(By contrast, my father-in-law went to university in Tokyo - he can just switch to standard Japanese at will. I have no trouble at all with him.)
ft