Before you change stat and anti freeze, I would run around for a few hundred miles with WYNN's cooling system cleaner, it gets out every last bit of crap out of there, and will leave it in the bottom of the expansion tank in a opaque gel like substance, but god it works so well.
Then new stat/a,freeze and if still reading low a temp sender, mine always shows low about 1/4 even with new stat etc, though if it sits in traffic and goes up to about 3/4 it will generally stay there for the rest of the day.
tabetha