You may want to keep the heat lamp set on one end of their brooder box, that way they have a warm section to go to when they feel chilled. Monitor the temps of the whole brooder, if it's already in the 90's in the garage they sure won't need any extra heat during those times. Their behavior may tell you more than a thermometer, if they're huddled under the light they need more heat, if they're staying far away & panting they need less. If they're happily peeping & pecking & moving easily around the box they are just right.
I've never used sawdust for bedding, just straw or non-cedar shavings, so I wouldn't know how that could harm them. I guess you could check to see if their crops feel hard or packed, that might mean they're eating too much sawdust. You could change to a different bedding if you want to eliminate that concern.
If the others are acting lethargic you could try giving them some of the vitamins, or put a little sugar in their water, or give them some hard-boiled egg. If they're otherwise acting well you may not need that.
It still could just be something inherently wrong with the chick, that happens often in the best of circumstances. It's Nature's Way that the hen lays more eggs than she needs to have hatch, hatches more chicks than she needs to grow, grows more chicks than are needed to survive to adulthood. But whenever it happens, it helps to look at all the other variables to make sure there isn't something else contributing to the loss.