Having covered 2800 miles round France in my Zed over the past fortnight with extensive use of the nav, I can confirm these "quirks". Typically, they always appear when you need it most, such as on a roundabout or approaching motorway exits. It also seems to "update" the route quite often, with the yellow one-screen route completely disappearing until it's recalculated.
However, one thing I can definitely confirm is that it really doesn't help if you store one of those reflective sheets for windscreens across the boot. Seems to block the antenna almost completely!!!