Extremely strange. The only thing i would suggest is ensuring that you are putting the case on it does not short on anything. Is the case touching any of your components wires etc etc.
As you have already gone through the standard H/W elimination technique i assume this is already done.
I DO NOT reccomend this but...if you turn it on with the case open, can you then attach the enclosure? Does it then turn off?
It freezes my system dead when I do that