At this age, those chicks should have about 1.5 to 2 s.f./bird. That's about 30 - 40 s.f. or 5 x 6 to 5 x 8. I'd turn the light off. At 8 weeks, they don't need it. No way to tell why they are dying without an autopsy.
How big is the brooder box, what is the temp. These chicks are almost 8 weeks old, and should have been off heat several weeks ago. What is the ambient temp in the rest of your garage? I assume that the chicks are fully feathered now? When you find the dead ones, are they in the corner? By...