Problems with a steam iron

I recently bought a brand new Black & Decker Steam Advantage iron (model
F2200). This is one with a "Ceramic Glide" soleplate. I've owned it less
than a week and I am already having problems with it. The specific problem
I am having is that on occasion it is staining my clothes. This happens
when I am ironing over a thick seam, say, the outside corner of the pocket
on the front of a shirt. The iron is leaving a thin, brown stain there.
Sometimes I can brush off what seems to be some residue there, but sometimes
not.

When I examine the soleplate I see some brown stains immediately below the
"top" steam vent, extending in a V-shape in each direction. This stained
area feels slightly rough, as though there is residue there.

Compared with my old iron, which had a non-stick (Teflon?) soleplate, I've
noticed that the ceramic soleplate of this new iron is more "sticky," and I
have to let the steam flow really build up before I can move the iron back
and forth easily. This soleplate also seems to get hotter at a given fabric
heat/steam setting.

I have ironed almost exclusively dress shirts, both 100% cotton and
poly/cotten shirts, with this iron. I am using the recommended heat/steam
setting for the fabric. I have been filling the iron with store-bought,
distilled water.

I have two questions:

(1) What is causing this problem? Is the recommended setting too hot? Is
this residue microscopic, burned, fabric particles?

(2) How can I clean this material off the soleplate?

Thanks in advance. I saw from searching Google that problems with steam
irons have been discussed in this group in the past.