Separate rack for the gear....
I used to have an enclosed rack. Never again...
Simple test - pull the PS3 out and see if it does the same thing while outside the cabinet. My fan comes on all the time too, but always in low speed mode, and it is likely that there isn't a time when the fan won't come on after the unit has been on for about 5-10 minutes. They sell add on cooling packs too. Did it do that before?