I will agree with too much heat, (1) 100W is plenty. But, something isn't adding up here. What type of behavior are the other chicks displaying? What about the ones that were dying? Did they appear to be panting or moving away from the light? I just had a batch that seemed especially sensitive to the light, but by watching them they pretty much let you know. I had them in my backroom and the setting sun was hitting their brooder in addition to the light and they were all in there panting away. So I turned the light off and moved them away from the sun. However, they were just fine. It sounds maybe more like something is systematically killing your chicks. If were just too hot, I would think the other would be panting. I am just saying, seems fishy.
I would check their feed for any type of moisture problem before you got it, does it smell musty or moldy? What kind of water are you giving them, is it a high salt well water? Mine is, so I give mine Osmosis water. Have you called the store to see if anyone else called with similar problems? There are tons of problems, environmental, viral, bacterial that could be affecting your chicks. They are generally pretty tough from my experience, I don't think your are doing anything wrong that would be causing their death. I never use a thermometer to manage my chicks heat, I move it according to their behavior. Are they huddled under it, moving away from it, seem generally content, panting..etc. I don't think 5 degrees is what is killing them.
Are they somehow eating something other than chicken feed? You mentioned using chick grit. I have never used chick grit when they are on strictly starter crumbles. I never feed my chicks ANYTHING but theor crumbles until they are well started and then I introduce grit and let the free range, give scraps etc. That's just me though. I would remove the chicks from the brooder and clean it very well. Once it is clean, Lysol the heck out of it and wipe it out with clean paper towels. Let is air dry. Check the bedding you are using for mustiness as well.
Could be about anything. If it were me I would try an antibiotic (goes in their water) from the feed store (as a last resort). Also, take precautions to protect yourself as well. Be sure to wash your hands after handing the chicks, coop, brooder, etc. Can't rule out something like Salmonella. That is a silent, quick chick killer.
I am real sorry for your loss. Not a good first time experience. Not to make light or mean any disrespect, but you are experiencing the hardest part first, and you are gaining valuable experience. I hope that you figure out what is wrong, normally things go a lot more smoothly. Keep us posted and good luck.