Is your cabinet open to the front or rear? The 3805 will get hot, especially if there's a lack of ventilation. I had the same problem because the front of my entertainment system has glass doors, and the rear had holes for the interconnects.
The receiver was too hot to touch. I mounted a cpu fan over the heat sink, which helped a ton. It still was pretty warm, so I completely removed the back panel of the entertainment system. Now it never gets more than warm. You can't beat ventilation. 90 degrees seems ok BTW.