I will usually put a thermometer at the COLD end of the brooder to see if they can adequately escape the heat. Sometimes it surprises me how warm it is 24" away from the lamp.
Any incessant cheeping?
You mentioned everyone was pooping OK, I'd double check that to make sure no pasty butt. We have a hen that consistently gives us a couple of chicks a week that have problems. A couple of them have had a very thin layer of poop blocking the vent and causing problems, not like the usual mound on the rear.
Sometimes chicks just die once their yolk sac meal is up (day 3ish) due to internal problems that we could never diagnose before they die.
Don't ever trust a single thermostat, especially one that is built it to anything. The more the merrier(and more accurate)!
Hopefully you get it figured out.