I had a simlar problem in a box that was 5' tall, 30" across and 28" deep. Had a 250w hps and a daytona 150cfm fan. Temps soard into the 90's. First I made sure that the box wasn't pushed up against a wall. This helped giving more free flowing cool air, but still the temps were mid to upper 80's. So then I got an idea. I screwed a couple furing strips just below the lamp, and dropped a piece of clear tempered glass in there. I wasn't a tight fit, and you don't want it to be so you get good air circulation around the plants... but it isolated the lamp and the ballist enough that my temps dropped down to the mid 70's then.

Just a suggestion....

Jaxxx