does it have a heatsheild on the back?its a chunk of metal made to absorb all the heat away from the unit,if its a mad unit it may not have been fitted when u installed it,may be loose in the packaging??
If its not that then is it cramped in the area its fitted,are the heater pipes back there that are making its home hot?
Maybe try taking it out and running it for an hour just sat on the centre console infront of the gearknob?? Are u playing it at really loud volume,i know my headunit gets mega hot if i crank it up for any extended legnths of time,its never cut out on me tho