I agree with Bigtopsfinn, You might be nute burning, not heat burning. The only way I've seen heat burn with my CFLs is to have them actually touching the CFL.

You want your grow space to sit in the 70-78F range roughly. So if your box is getting to hot then add fans on the top of the box(blowing out). Then drill vent holes in the bottom(because heat rises).

That will also pull more CO2 enriched air through the plant/s, while keeping it cooler:thumbsup:.